conf: add energytune to domain XML

conf: add energytune to domain XML

The new XML element is <energytune> under <cputune> following earlier pattern for
resctrl features (cachetune, memorytune). Energytune doesn't currently support
the "tuning" part, only monitoring. I added it as energytune for consistency with
cache and memory features, keeping all resctrl handling under cputune. This also makes
sense with current resctrl architecture - all monitoring groups are part of an
allocation group.

Changes:
 - Added <energytune> parsing to domain_conf.c
 - Added schema definition in domaincommon.rng
 - Documented the element in formatdomain.rst
 - Added energytune test

Signed-off-by: Jedrzej Wasiukiewicz <jedrzej.wasiukiewicz@intel.com>
Signed-off-by: Christopher M. Cantalupo <christopher.m.cantalupo@intel.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>