Re: [libvirt] [PATCH v6 13/17] [ACKED] qemu: add a USB3 controller to Q35 domains by default

Previously we added a set of EHCI+UHCI controllers to Q35 machines to mimic real hardware as closely as possible, but recent discussions have pointed out that the nec-usb-xhci (USB3) controller is much more virtualization-friendly (uses less CPU), so this patch switches the default for Q35 machinetypes to add an XHCI instead (if it's supported, which it of course *will* be).   Since none of the existing test cases left out USB controllers in the input XML, a new Q35 test case was added which has *no* devices, so ends up with only the defaults always put in by qemu, plus those added by libvirt.