Make disks attribute path required check for name collisions (#1293)

Ensure the path attribute for disks in the new format is required to
ensure there is always one parameter required for the hash element in
the array of disks. This avoids a certain amount of magical behaviour.

Identify that the name attribute is optional, however add checks for
collisions in volume names generated, as well as ensuring the volumes
are still prefixed in a way to prevent accidental collisions between
boxes utilizing the same names.

Add notes to the README identifying the format as experimental, with
notes on how the format should appear.

README.md

@@ -63,7 +63,9 @@ can help a lot :-)
 * [Memory balloon](#memory-balloon)
 * [Libvirt communication channels](#libvirt-communication-channels)
 * [Custom command line arguments and environment variables](#custom-command-line-arguments-and-environment-variables)
-* [Box Format](#box-format)
+* [Box Formats](#box-formats)
+  * [Version 1](#version-1)
+  * [Version 2 (Experimental)](#version-2-experimental)
 * [Create Box](#create-box)
 * [Package Box from VM](#package-box-from-vm)
 * [Troubleshooting VMs](#troubleshooting-vms)
@@ -1774,7 +1776,11 @@ Vagrant.configure("2") do |config|
 end
 ```
 
-## Box Format
+## Box Formats
+
+### Version 1
+
+This is the original format that most boxes currently use.
 
 You can view an example box in the
 [`example_box/directory`](https://github.com/vagrant-libvirt/vagrant-libvirt/tree/master/example_box).
@@ -1788,6 +1794,45 @@ The box is a tarball containing:
 * `Vagrantfile` that does default settings for the provider-specific
   configuration for this provider
 
+
+### Version 2 (Experimental)
+
+Due to the limitation of only being able to handle a single disk with the version 1 format, a new
+format was added to support boxes that need to specify multiple disks. This is still currently
+experimental and as such support for packaging has yet to be added. There is a script in the tools
+folder (tools/create_box_with_two_disks.sh) that should provide a guideline on how to create such
+a box for those that wish to experiment and provide early feedback.
+
+At it's most basic, it expects an array of disks to allow a specific order to be presented. Disks
+will be attached in this order and as such assume device names base on this within the VM. The
+'path' attribute is required, and is expected to be relative to the base of the box. This should
+allow placing the disk images within a nested directory within the box if it useful for those
+with a larger number of disks. The name allows overriding the target volume name that will be
+used in the libvirt storage pool. Note that vagrant-libvirt will still prefix the volume name
+with `#{box_name}_vagrant_box_image_#{box_version}_` to avoid accidental clashes with other boxes.
+
+Format and virtual size need no longer be specified as they are now retrieved directly from the
+provided image using `qemu-img info ...`.
+
+Example format:
+```json
+{
+  'disks': [
+      {
+          'path': 'disk1.img'
+      },
+      {
+          'path': 'disk2.img',
+          'name': 'secondary_disk'
+      },
+      {
+          'path': 'disk3.img'
+      }
+  ],
+  'provider': 'libvirt'
+}
+```
+
 ## Create Box
 
 If creating a box from a modified vagrant-libvirt machine, ensure that