Add <hostdev> protocol=vhost model handling for virtio transitional
devices. Ex:

  <hostdev mode='subsystem' type='scsi_host' model='virtio-transitional'>
    <source protocol='vhost' wwpn=X/>
  </hostdev>

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