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

  <vsock model='virtio-transitional'>
    ...
  </vsock>

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