MB-19746: Add xml output to ep_perfsuite
MB-19746: Add xml output to ep_perfsuite

- The new performance commit validation, CBNT, requires input data
  in XML format which mirrors that of the GTest XML output
- This commit adds this functionality to ep_perfsuite, providing
  the ability to output the results in textual format or xml
- It can be invoked by using the new `-f` flag, followed by either
  `xml` or `text`, if this flag is not specified then the default
  behaviour of textual output is assumed

Change-Id: I153a816493181acb091e8d4433f5be543742231f
Reviewed-on: http://review.couchbase.org/65005
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,teligent-ru/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,membase/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/ep-engine,couchbase/ep-engine,daverigby/kv_engine,owendCB/ep-engine,owendCB/ep-engine,couchbase/ep-engine,membase/ep-engine,owendCB/ep-engine,daverigby/kv_engine,daverigby/ep-engine,membase/ep-engine,couchbase/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine