if the call is invoked through bdrv_is_allocated the caller might
expect *pnum = 0 on error. however, a new implementation of
bdrv_get_block_status might only return a negative exit value on
error while keeping *pnum untouched.
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NPeter Lieven <pl@kamp.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>