Rework device-path handling

Pass full device paths around, rather than shortened device names and
mountpoints.

The parse() function has been updated to only take a device path; the
mountpoint can be derived with mountpoint_for_device. Also, add
initial handling for uuid= and label= syntaxes.

This allows us to remap ps3 devices where necessary.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>