Add new <interface> model handling for virtio transitional devices. Ex:

<interface>
  <model type='virtio-transitional'/>
</interface>

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