QPID-4519: return true for VirtualHost MBean isStatusEnabled, dont update stats when doing so, and stop using a synchronized method as a result

merged from trunk r1424763


git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.20@1430320 13f79535-47bb-0310-9956-ffa450edef68