core: Move MultiValueMapUtils

Moved MultiValueMapUtils from org.ovirt.engine.core.utils to
org.ovirt.engine.core.utils.collections, where all the map-related
utilities reside.

Change-Id: I4d4b7278e43dd4bf5700c3b65e73833c3bda43dc
Signed-off-by: Allon Mureinik <amureini@redhat.com>