QPID-5834: [Java Broker] Move validation that prevents to deletion of default virtualhost to validate, in order that it can veto the proposed state change

* Added a unit test, improve other VH unit tests
* Removed MockVirtualHost

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1605011 13f79535-47bb-0310-9956-ffa450edef68