Add <controller type='scsi' model handling for virtio transitional
devices. Ex:

  <controller type='scsi' model='virtio-transitional'/>

* "virtio-transitional" maps to qemu "virtio-scsi-pci-transitional"
* "virtio-non-transitional" maps to qemu "virtio-scsi-non-transitional"

The naming here doesn't match the pre-existing model=virtio-scsi.
The prescence of '-scsi' there seems kind of redundant as we have
type='scsi' already, so I decided to follow the pattern of other
patches and use virtio-transitional etc.
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>