Add more virtualization images

Adds a bunch of disk image formats used in virtualization.

Unfortunately none of these have official registered media types, and for all but OVF I had to invent one. OVF has an actual spec with /ovf in it, but that's not in IANA either. There are some x-virtualbox-* ones for these around, but these aren't virtualbox specific, so I think we want a more generic one. Followed the existing x-foo-disk practice from qcow in the names -- not that happy about it, but at least somewhat consistent. No that strong opinions about the naming, improvement ideas welcome.