throttle: refuse bps_max/iops_max without bps/iops

The bps_max/iops_max values are meaningless without corresponding
bps/iops values.  Reported an error if bps_max/iops_max is given without
bps/iops.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 1438683733-21111-2-git-send-email-stefanha@redhat.com