Consider using --subvolid when deleting subvolumes

[vojtechtrefny]

Not to suggest a change in strategy, but just extra info: it is possible to delete subvolumes without "line of sight" via path-to-suvolume. One is btrfs subvolume delete --subvolid <subvolid> option. Less obvious, is the example in libbtrfsutil, both C API and Python bindings have an _fd variant for removing subvolumes. There's also a recursive delete subvolume option, not available in the CLI commands.

Originally posted by @cmurf in #1015 (comment)

[vojtechtrefny]

Thanks, this is definitely something we should consider doing in the future. We are also planning to switch libblockdev to use libbtrfsutil (storaged-project/libblockdev#552) so that might be also a good opportunity to change few things in the btrfs implementation in Blivet.