block: Assert no write requests under BDRV_O_INCOMING As long as BDRV_O_INCOMING is set, the image file is only opened so we have a file descriptor for it. We're definitely not supposed to modify the image, it's still owned by the migration source. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com>