core: use Collections#EMPTY_MAP instead of new map

In VmHandler#findStorageDomainForMemory(Guid, long) we create everytime
a new map and pass it on. This new map is empty map and no one adds
anything to it, so we can use the singleton map Collections#EMPTY_MAP
instead.

Change-Id: I4053e06f17827d8afbdc48ad006897c029426384
Signed-off-by: Arik Hadas <ahadas@redhat.com>