https://bugzilla.redhat.com/show_bug.cgi?id=1128751

There's this <driver/> element under <interface/> which can have
several attributes. However, the driver element is currently formated
only if the driver's name or txmode has been specified. This makes
only a little sense as we parse even partial <driver/>, for instance:

    <interface type='user'>
      <mac address='52:54:00:e5:48:58'/>
      <model type='virtio'/>
      <driver ioeventfd='on' event_idx='on' queues='5'/>
    </interface>

But such XML would never get formatted back.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>