[jvstm-cluster] Add getNumMembers
[jvstm-common] Add method getId to VBox
[jvstm-common] Make VBox abstract (it was already used as such)

The VBox class is never instantiated.  So we mark it as such.
[jvstm-common] Add OwnedVBox (previously VBox)

  * Abstract VBox ID.  Use the id instead of explicitly assuming the existence
    of a slotname and an ownerId.  OnwedVBox defines its ID based on these
    fields.
  * add method findVBox given its id: looks up a cached vbox.  Depending on
    the backend, the implementations will either use a VBox cache or the
    DomainObject cached (e.g. when the VBox is an OwnedVBox).