Allow to use many disks in vagrant box for libvirt provider

Adds support for a new multi disk box format and handling to upload the
multiple disks to the storage pool.

New format is:
{
  'disks': [
    {
      'name': 'disk1.img',
      'virtual_size': 10,
      'format': 'qcow2'
    },
    {
      'name': 'disk2.img',
      'virtual_size': 15,
      'format': 'qcow2'
    },
    {
      'name': 'disk3.img',
    }
  ],
  'provider': 'libvirt',
  'format': 'qcow2'
}

It is expected to remove format from being set at the top level when
using the new format, with the assuming that qcow2 should be the default
format, and other formats should be permitted to be specified as needed.

Includes tests for handling the box images and creation of domain
volumes. Additionally includes an integration test to ensure a box with
2 disks will work as expected.

Partially fixes: #602

.github/workflows/integration-tests.yml

@@ -18,6 +18,7 @@ jobs:
         - simple vm provision via shell
         - bring up with custom default prefix
         - bring up with second disk
+        - bring up with two disks
         - bring up with adjusted memory settings
         - bring up with adjusted cpu settings
         - ip is reachable with private network