[libvirt] Add USB controller support

[Christophe Fergeau]

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