jcr:uuid property in virtual nodes tree should not be existing

This because now a UUID appears multiple times in the overall tree. The jcr:uuid should be restricted to the real nodes, mirror nodes can have a hippo:uuid which just happens (deliberatly to our model) to have the same value as the jcr:uuid.

The getCanonicalNode method in HippoNode uses this property, however this method is yet unused, hence initial priority.