vsperf: Add --xunit support

vsperf: Add --xunit support

Generate xmlrunner output for use by Jenkins, but without actually
using xmlrunner to discover & run tests.

JIRA: VSPERF-27

Change-Id: I000099c841f3755043104f251837e3d4c214c6bb
Signed-off-by: Billy O'mahony <billy.o.mahony@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Eugene Snider <Eugene.Snider@huawei.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Gurpreet Singh <gurpreet.singh@spirent.com>
Reviewed-by: Tv Rao <tv.rao@freescale.com>