Enable XML sar data collection

Add XML data collection via sadf -x to the sar plug-in to allow
the collected performance data to be imported by other tools on
the analysis host.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>

Conflicts:
	sos/plugins/sar.py
Imrpove preamble text content and templating

Partial backport of 7ccd5f6.

Signed-off-by: Bryn M. Reeves.