libblockdev has historically used btrfs-progs CLI utilities in the
meanwhile libbtrfsutil has been developed and used by snapper.

Not parsing CLI output and using the library functions should provide a
more stable way to gather information from btrfs.

libbtrfsutil provides it's own error enum for libbtrfsutil functions and
as we no longer parse output this introduces a BD_BTRFS_ERROR_NOT_FOUND
error type. As the mount point not being found is the most likely
failure scenario.

Use libbtrfsutil's subvolume iterator to obtain a list of subvolumes in
order. In the future this would allow exposing more information about a
subvolume to list_subvolumes.

Avoids having to exec btrfs-progs and allows us to implement recursive
subvolume deletion in the future.