ppc: Over ppc arch vdsm reports UNKNOWN as operationSystem in getVdsCaps

Vdsm currently checks only /etc/redhat-release path which exists over
fedora installation for ppc arch but not on PowerKVM platforms.
Over PowerKVM platforms we should read /etc/ibm_powerkvm-release to
retrieve the release version.

Change-Id: I5e6ef83353b4b47a22d91d222ef1a0122b9470d3
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1131965
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32444
Reviewed-by: Dan Kenigsberg <danken@redhat.com>