throttle: refuse iops-size without iops-total/read/write

In a similar vein to commit ee2bdc33
("throttle: refuse bps_max/iops_max without bps/iops") it is likely that
the user made a configuration error if iops-size has been set but no
iops limit has been set.

Print an error message so the user can check their throttling
configuration.  They should either remove iops-size if they don't want
any throttling or specify one of iops-total, iops-read, or iops-write.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 1464828031-25601-1-git-send-email-stefanha@redhat.com