use dev_t to identify mounts, not device files

use dev_t to identify mounts, not device files

This should fix all issues with things like device-mapper using
non-canonical device file names (e.g. /dev/mapper/VolGroup00-LogVol00).
Also, this allows us to delete a bunch of special case code that
already tried to deal with this

See https://bugzilla.redhat.com/show_bug.cgi?id=488258 for details.