perf regression: save result to xml for reporting to jenkins
perf regression: save result to xml for reporting to jenkins

This patch saves performance results to xml file, it will be
parsed by jenkins plugin (PerfPublisher) in the end of job,
then we can check and compare history results from jenkins.

Reference:
https://wiki.jenkins-ci.org/display/JENKINS/PerfPublisher+Plugin

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests