QPID-5803: [Java Broker] Add a utilised flag to the VHN context after the blueprint has been used.

This prevents the virtualhost from being automatically recreated by the blueprint, should the user choose to the VH.
Strenghthen unit tests.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1604164 13f79535-47bb-0310-9956-ffa450edef68