qcow2: Implement .bdrv_inactivate The callback has to ensure that closing or flushing the image afterwards wouldn't cause a write access to the image files. This means that just the caches have to be written out, which is part of the existing .bdrv_close implementation. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com>