Add mtu option for private networks and interfaces

In order to create virtual networks with different MTU sizes
than the libvirt default of 1500, this patch adds a
libvirt__mtu option. This will add an mtu option to the libvirt
network definition and to the domain interface definition as well.

README.md

@@ -626,6 +626,9 @@ starts with `libvirt__` string. Here is a list of those options:
   failures](https://github.com/vagrant-libvirt/vagrant-libvirt/pull/498)
 * `: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
   create a domain value by default virtio KVM believe possible values, see the
   documentation for libvirt

lib/vagrant-libvirt/action/create_network_interfaces.rb

@@ -76,6 +76,7 @@ module VagrantPlugins
             @driver_name = iface_configuration.fetch(:driver_name, false)
             @driver_queues = iface_configuration.fetch(:driver_queues, false)
             @device_name = iface_configuration.fetch(:iface_name, false)
+            @mtu = iface_configuration.fetch(:mtu, nil)
             template_name = 'interface'
             # Configuration for public interfaces which use the macvtap driver
             if iface_configuration[:iface_type] == :public_network
@@ -143,6 +144,7 @@ module VagrantPlugins
                                     @device_name,
                                     @iface_number,
                                     @model_type,
+                                    @mtu,
                                     driver_options,
                                     @udp_tunnel)
                     else
@@ -234,7 +236,7 @@ module VagrantPlugins
         end
 
         def interface_xml(type, source_options, mac, device_name,
-                          iface_number, model_type, driver_options,
+                          iface_number, model_type, mtu, driver_options,
                           udp_tunnel={})
           Nokogiri::XML::Builder.new do |xml|
             xml.interface(type: type || 'network') do
@@ -245,6 +247,7 @@ module VagrantPlugins
               xml.target(dev: target_dev_name(device_name, type, iface_number))
               xml.alias(name: "net#{iface_number}")
               xml.model(type: model_type.to_s)
+              xml.mtu(size: mtu) if mtu
               xml.driver(driver_options)
             end
           end.to_xml(

lib/vagrant-libvirt/action/create_networks.rb

@@ -286,6 +286,8 @@ module VagrantPlugins
           @network_bridge_name = @interface_network[:bridge_name]
           @network_address = @interface_network[:ip_address]
           @network_netmask = @interface_network[:netmask]
+          @network_mtu = @options[:mtu]
+
           @guest_ipv6 = @interface_network[:guest_ipv6]
 
           @network_ipv6_address = @interface_network[:ipv6_address]

lib/vagrant-libvirt/templates/private_network.xml.erb

@@ -6,6 +6,10 @@
     <domain name="<%= @network_domain_name %>" localOnly="yes" />
   <% end %>
 
+  <% if !@network_mtu.nil? %>
+  <mtu size="<%= @network_mtu %>" />
+  <% end %>
+
   <% if (@network_forward_mode != 'none' && @network_forward_mode != 'veryisolated') %>
     <% if @network_forward_device %>
       <forward mode="<%= @network_forward_mode %>" dev="<%= @network_forward_device %>" />