Allow change volume title of ISO image from DVDVIDEO

[JohnVeness]

At the moment, all ISOs created by devedeng are given the volume title DVDVIDEO (this is hard-coded in genisoimage.py). Given that the volume title is visible on some occasions, especially when playing DVDs on a computer, or on a games console, it would be good if users could change that title to one of their own choosing.