It is not possible to remove rollback snapshot if it contains other subvolumes

Currently we create rollback snapshots by moving existing @ to rollback one. The problem is that if there are some subvolumes created inside we move those as well. These subvolumes are kept in that snapshot which can be even considered correct but such snapshot can't be removed simply by btrfs subvol delete because it reports that snapshot is not empty.

We should instead of rollback possibly do just plain snapshot and drop subvolumes or possibly remove snaphosts recursively.

Just to point out how those snapshots were created. LXC creates snapshots so it is enough to run LXC on schnapps managed drive to reproduce this.