Improve dvdnav_get_volid_string API (avoid free)

Using the recently introduced dvdnav_get_volid_string implies that the calling application has to manually free the string. IMHO this can be seen as a sign of bad API design and the usage itself is cumbersome (e.g. the const_cast here because the libdvdnav API returns a const char*). Since this is not yet released as part of any public release let's try to improve it, allowing it to be part of the vm struct and destroyed on Close() just like dvd_name and dvd_serial.