pybind/mirroring: add subvolume/subvolumegroup interfaces for snapshot mirroring
Rather than the operator adding subvolume/subvolumegroup paths via "fs snapshot mirror add/remove" interface, introduce subvolume/subvolumegroup interfaces. mgr/mirroring can resolve subvolume/subvolumegroup to filesystem paths internally.
Note: mgr/volumes v1 and v2 subvolumes have different snap locations, i.e., v1 snaps are under /volumes/<group>/<subvolume>/<uuid> and v2 snaps are under /volumes/<group>/<subvolume>. Auto upgraded subvolumes (no version -> v1) also needs to be taken into consideration.