webadmin: UnitVmModel subscribe GetSPClusterService

UnitVmModel - Subscribe VdcQueryType.GetStoragePoolsByClusterService;
introduced in http://gerrit.ovirt.org/#/c/11395/

The query subscription is needed for progress mechanism:
prevents hourglass 'blinking'.

Change-Id: I7c2324a8569cc4c96b89a6a62fa4764c1518adb1
Signed-off-by: Daniel Erez <derez@redhat.com>