Auto-detect subvolume mountpoints within volume paths

[tasket]

Problem:

Currently Wyng requires references to local Btrfs storage to be a subvolume mountpoint. However, this ignores the possibility that other subvolumes may be nested along the sub-paths of one or more disk image files; any image file within such a 'sub-subvolume' path would not actually reside in the --local subvolume that Wyng is trying to snapshot and Wyng would probably halt with errors during delta acquisition.

Expecting the user to always have their disk image directory free of additional subvols may be too much to ask.

Solution:

Get a complete list of subvolume mounts for the referenced filesystem and see if any match the paths of the disk images. Then possibly have get_reflink_deltas() call itself recursively to handle the imgs in the embedded subvols.

Also, bind-mounts will have to be detected.

Workarounds:

• Move or reflink the failing disk image to a path within the referenced --local subvol.
• Exclude the failing/nested disk image from the main backup session, then run another session with --local pointing instead to the nested subvol.