[libvirt] USB redirection support

Hey, This patch series adds the needed classes for apps to be able to use USB redirection. I've followed http://hansdegoede.livejournal.com/11084.html and added all the API needed to be able to get the same XML. However, I'm not 100% sure all these attributes are required, in particular the various addresses. Given that for now libvirt-gconfig users tend to reuse what test-domain-create does, it would be useful to know which parts are required and which are not to get working USB redirection. Thanks, Christophe