qemu-iotests: Fix 232 for non-qcow2

232 is marked as generic, but commit 12efe428 added code that assumes
qcow2. What the new test really needs is backing files and support for
updating the backing file link (.bdrv_change_backing_file).

Split the non-generic code into a new test case 247 and make it work
with qed, too.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>