iperf/jenkins integration

Add a script to convert an iperf bandwidth measurement to an xml format
suitable for the jenkins perfpublisher plugin [1].

At the moment, only a single test is supported, but that will be extended
later.

[1] https://wiki.jenkins-ci.org/display/JENKINS/PerfPublisher+Plugin

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>