Re: [PATCH v3 1/4] block: Add trivial backing_fmt support to qcow, sheepdog, vmdk

For qcow2 and qed, we want to encourage the use of -F always, as these formats can suffer from data corruption or security holes if backing format is probed. But for other formats, the backing format cannot be recorded. Making the user decide on a per-format basis whether to supply a backing format string is awkward, better is to just blindly accept a backing format argument even if it is ignored by the contraints of the format at hand. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt;