Use the volume id of DVD for media-title property

Currently, there's no way to retrieve DVD volume id which contains human-readable string.
This patch adds a new stream control, STREAM_CTRL_GET_DVD_VOLUME_ID to stream_dvd and stream_dvdnav to obtain DVD volume id.
Furthermore, I think this is proper value as media-title for DVD(at least, it's better to display volume id than device name as media-title), so I made some change for media-title property to return DVD's volume id if available.