qemu: Support rng model=virtio-{non-}transitional

qemu: Support rng model=virtio-{non-}transitional

Add new <rng> model values for virtio transitional devices. Ex:

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

* "virtio-transitional" maps to qemu "virtio-rng-pci-transitional"
* "virtio-non-transitional" maps to qemu "virtio-rng-pci-non-transitional"

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>