[pve-devel] pve-storage : volume_clone

[Alexandre Derumier]

This add volume_clone sub for storage which support it.

for file, I set parent file in readonly, so we can't delete or modify it if clone exist.
rbd have a lock to protect snapshot from deletion
nexenta has also a lock on the cloned snapshot
sheepdog don't have lock, but we can safely delete a parent snapshot or image, without breaking the clone.