core: fix changing a VM cluster

Due to the latest refactoring of VM entity the check for updating
vds_group_id in ChangeVmClusterCommand is no longer valid.

Modified vds_group_id vdsGroupId.

Verified with DOWN VMs.

TODO the weak implementation of VMHandler should be changed.

Change-Id: I3a92158800571358bf59f15972f811f7c0e9d3fe
Signed-off-by: Roy Golan <rgolan@redhat.com>