ovs-ofctl: Make "barrier" output reproducible for testing.

ovs-ofctl: Make "barrier" output reproducible for testing.

The "ofproto - asynchronous message control" test had a race in which
the "send: OFPT_BARRIER_REQUEST" message could get printed in different
places because there was nothing to ensure that messages from the switch
were printed before messages sent to the switch, even though the actual
ordering of the messages was predictable.  This fixes the problem by not
printing a message at all when the barrier request is sent.

Bug #10049.
Signed-off-by: Ben Pfaff <blp@nicira.com>