Handle nic model type updating during start (#1747)

Start domain added some support to update the nic model type if it
changed to avoid needing to completely destroy and recreate the domain.
Additionally support for setting the management network nic model type
was added, however the default value did not correctly reflect expected.

This updates `management_network_model_type` to default to
`nic_model_type`, which in turn defaults to `virtio`. This has now been
moved from the create network interfaces action into the config object
to allow chained resolving. This should ensure that setting
`nic_model_type` will correctly cause all networks to use the model
unless explicitly specified to use a different model.

Additionally the start domain action for updating the interfaces has
been modified to loop over all interfaces and all configured adapters in
the order they should have been added. This allows for checking whether
the model type needs to be updated based on the
`management_network_model_type` config option for the first network,
assuming the management network has been attached, and otherwise to fall
back to the default from `nic_model_type` for all other interfaces.

Finally ensure the iommu attribute is removed from the nic driver if
the model type is switched to anything other virtio to avoid issues with
attempting to enable invalid settings.

Fixes: #1743

docs/configuration.markdown

@@ -102,9 +102,7 @@ end
     Note: for discard to work, you will likely also need to set `disk_bus = 'scsi'`
   * `:detect_zeroes` - Controls whether to detect zero write requests. The value can be "off", "on" or "unmap".
   * `address_type` - Address type of disk device to emulate. If unspecified, Libvirt uses a sensible default.
-* `nic_model_type` - parameter specifies the model of the network adapter when
-  you create a domain value by default virtio KVM believe possible values, see
-  the [documentation for Libvirt](https://libvirt.org/formatdomain.html#setting-the-nic-model).
+* `nic_model_type` - parameter specifies the model of the network adapter when you create a domain, default is 'virtio'. For possible values, see the [documentation for Libvirt](https://libvirt.org/formatdomain.html#setting-the-nic-model).
 * `shares` - Proportional weighted share for the domain relative to others. For more details see [documentation](https://libvirt.org/formatdomain.html#cpu-tuning).
 * `memory` - Amount of memory in MBytes. Defaults to 512 if not set.
 * `cpus` - Number of virtual cpus. Defaults to 1 if not set.
@@ -498,12 +496,12 @@ starts with `libvirt__` string. Here is a list of those options:
   failures](https://github.com/vagrant-libvirt/vagrant-libvirt/pull/498). Note
   that you cannot use names reserved for libvirt's usage based on [documentation](
   https://libvirt.org/formatdomain.html#overriding-the-target-element).
-* `:mac` - MAC address for the interface. *Note: specify this in lowercase
+* `:libvirt__mac` - MAC address for the interface. *Note: specify this in lowercase
   since Vagrant network scripts assume it will be!*
 * `:libvirt__mtu` - MTU size for the Libvirt network, if not defined, the
   created network will use the Libvirt default (1500). VMs still need to set the
   MTU accordingly.
-* `:model_type` - parameter specifies the model of the network adapter when you
+* `:libvirt__model_type` - parameter specifies the model of the network adapter when you
   create a domain value by default virtio KVM believe possible values, see the
   documentation for Libvirt
 * `:libvirt__driver_name` - Define which network driver to use. [More
@@ -513,8 +511,8 @@ starts with `libvirt__` string. Here is a list of those options:
   info](http://www.linux-kvm.org/page/Multiqueue)
 * `:autostart` - Automatic startup of network by the Libvirt daemon.
   If not specified the default is 'false'.
-* `:bus` - The bus of the PCI device. Both :bus and :slot have to be defined.
-* `:slot` - The slot of the PCI device. Both :bus and :slot have to be defined.
+* `:libvirt__bus` - The bus of the PCI device. Both :bus and :slot have to be defined.
+* `:libvirt__slot` - The slot of the PCI device. Both :bus and :slot have to be defined.
 * `:libvirt__always_destroy` - Allow domains that use but did not create a
   network to destroy it when the domain is destroyed (default: `true`). Set to
   `false` to only allow the domain that created the network to destroy it.
@@ -592,7 +590,7 @@ used by this network are configurable at the provider level.
 * `management_network_keep` - Starting from version *0.7.0*, *always_destroy* is set to *true* by default for any network.
   This option allows to change this behaviour for the management network.
 * `management_network_iface_name` - Allow controlling of the network device name that appears on the host for the management network, same as `:libvirt__iface_name` for public and private network definitions. (unreleased).
-* `management_network_model_type` - Model of the network adapter to use for the management interface. Default is 'virtio'.
+* `management_network_model_type` - Model of the network adapter to use for the management interface. Default is `nic_model_type`, which in turn defaults to 'virtio'.
 
 You may wonder how vagrant-libvirt knows the IP address a VM received.  Libvirt
 doesn't provide a standard way to find out the IP address of a running domain.