qcow2: Fix header update with overridden backing file

In recent qemu versions, it is possible to override the backing file
name and format that is stored in the image file with values given at
runtime. In such cases, the temporary override could end up in the
image header if the qcow2 header was updated, while obviously correct
behaviour would be to leave the on-disk backing file path/format
unchanged.

Fix this and add a test case for it.

Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1428411796-2852-1-git-send-email-kwolf@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
