qcow2: Prohibit backing file changes in 'qemu-img amend'

qcow2: Prohibit backing file changes in 'qemu-img amend'

This was deprecated back in bc5ee6da7 (qcow2: Deprecate use of
qemu-img amend to change backing file), and no one in the meantime has
given any reasons why it should be supported.  Time to make change
attempts a hard error (but for convenience, specifying the _same_
backing chain is not forbidden).  Update a couple of iotests to match.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210503213600.569128-2-eblake@redhat.com>
Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>