LibVirt: Initial support for virtio-scsi virtual drives (read: ssd trim)

[alip]

User story

As an engineer I would like to add support for virtio-scsi devices in Libvirt.pm so I can automate usage of para-virtualized SCSI controller devices

Describe the solution you'd like

1. Honour driver_cache and driver_discard keys in storage disk config.
2. Add a SCSI controller if any of the storage disks have SCSI as the target bus.

Such that we could do e.g:

vm create => 'foo', 
    type    => 'kvm',
    storage => [
        {
             dev         => 'sda',
             bus         => 'scsi',
             driver_cache => 'writeback',
             driver_discard => 'unmap',
        }
    ],
    ...

Describe alternatives you've considered

N/A

Additional context

N/A