Add new <disk> model values for virtio transitional devices. When
combined with bus='virtio':

* "virtio-transitional" maps to qemu "virtio-blk-pci-transitional"
* "virtio-non-transitional" maps to qemu "virtio-blk-pci-non-transitional"
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>