Fix the XTS subsystem test

I added a test called XTSSubsystemTestCase, which uses our subsystem testing framework which attempts to validate the description providers, this will be committed to upstream in the next few days.
What I have added is a minimum of what is required for subsystem testing, the test framework allows you to do a lot of other stuff, see the jmx subsystem test for some examples.
The tests actually parse validation but fails a bit further down the line since the model does not seem to get populated. Hence I've @Ignored it for now.

The wip lives at https://github.com/kabir/jboss-as/tree/subsystem-test-model-fixes