Umm, /dev/ubd/X is the device in most cases (devfs on) and the whole disk is
also available as /dev/ubd/discX/disc with partitions /dev/ubd/discX/partY.
For the non-devfs case major = 98 while disc0 is minor 0 and disc1 is minor
16 disc0/part1 is 98,1 and disc1/part2 is 98,18
Actually you should be able to use LVM pretty much as you would on a host
system. UML now supports partitions (as of a long string of patches back to
about 2000) and you just fdisk, setup your partitions as desired and then
use (assumeing devfs) /dev/ubd/disc1/part1 /dev/ubd/disc2/part1 etc. to get
the partitions stored in the backing files.
If you are trying to use real hardware partitions you can add a second
partition table as above or try using the whole disk as a ubd device, I
would really suggest using files first though as they can be sparse and only
use space that is being used on the file system, and are much more forgiving
of errors, usually. trashing the wrong partition on a file is annoying,
trashing the wrong partition on a hard disk can be a disaster. Files are
also easy to backup.