core:SPM role should not be granted to Gluster only nodes

In a DC with gluster cluster and vdsm cluster, if the hypervisor
node with SPM role goes down, the SPM role should not be granted
to a gluster node.
Changed the function GetUpAndPrioritizedVds to check if virt_service
is true on the VDS.

Change-Id: I6054ba6ad9864798b5d914659f39edaf884e2215
Signed-off-by: Sahina Bose <sahina@gmail.com>