[libvirt] Add USB controller support

Hey, Here is the second version of the patch series implementing USB controller support in libvirt-gconfig. There's a bug fixed in GVirConfigDomainAddressUsb creation (type="usb" was missing) as well as a few small bug fixes/changes in the test program. I also added an additional patch implementing a gvir_config_domain_setup_default_usb_controllers to make adding usb controllers easier following Marc-André's suggestion. Christophe