This generates new XML like:

    <disk>
      <enum name='model'>
        <value>virtio</value>
        <value>virtio-transitional</value>
        <value>virtio-non-transitional</value>
      </enum>
    </disk>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>