Adding "updateVmPolicy" api

Adding an API method "updateVmPolicy".
The method will be called from the engine
when a VM SLA parameter will change, for now
it will support the change of vcpuLimit.

Change-Id: I9750667c4d20d7589a1797e65d5683692ec02afe
Signed-off-by: Kobi Ianko <kianku@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27272
Reviewed-by: Francesco Romani <fromani@redhat.com>
Tested-by: Martin Sivák <msivak@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>