snazzer-receive should cope with subvolume layout changes

[csirac2]

Example:

• Remote host has a subvolume /var/log. Parent dir var is an ordinary directory on a btrfs root.
• Remote host then changes /var to be a subvolume itself, while retaining /var/log subvol.
• snazzer-receive could be smart enough to detect this change and change the var directory on the receive destination to a subvol.
• Also consider how snazzer deals with other changes (Eg. subvolume moves/renames).
• This probably calls for development of proper test infrastructure/hooks.