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

  <input type='passthrough' bus='virtio' model='virtio-transitional'>
    ...
  </input>

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