When removeing support for qemu-img being able to create backing
chains without embedded backing formats, we caused a poor error
message as caught by iotest 114.  Improve the situation to inform the
user what went wrong.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210708155228.2666172-1-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>