iotests: Make 233 output more reliable

iotests: Make 233 output more reliable

We have a race between the nbd server and the client both trying
to report errors at once which can make the test sometimes fail
if the output lines swap order under load.  Break the race by
collecting server messages into a file and then replaying that
at the end of the test.

We may yet want to fix the server to not output ANYTHING for a
client action except when -v was used (to avoid malicious clients
from being able to DoS a server by filling up its logs), but that
is saved for a future patch.

Signed-off-by: Eric Blake <eblake@redhat.com>
CC: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190117193658.16413-2-eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>