core: remove VM from its Affinity Group on change cluster

For more information please refer
to: http://www.ovirt.org/Features/VM-Affinity

Change-Id: I0cc4f8ea49b4c79aea4d16239e09d2db8a07bd8a
Bug-Url: https://bugzilla.redhat.com/1036638
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>