Accept a new driver name attribute to specify usage of helper process, ex:

  <video>
    <driver name='vhostuser'/>
    <model type='virtio'/>
  </video>
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>