[libvirt] USB redirection support

[Christophe Fergeau]

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