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

  <memballoon model='virtio-transitional'/>

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