Update subvolume_info in btrfsutil to optionally take a UUID

[CopOnTheRun]

Currently the btrfsutil python binding has the option to get a SubvolumeInfo object from the subvolume_info function by passing either a path which will return the SubvolumeInfo object for that path, or if a subvolume ID is also provided, it will return the object matching that ID. The command line program also has the option to pass in a UUID (btrfs subvolume show --uuid ...), it would be nice to see this functionality mirrored in the python bindings as well.