qemu: auto-add generic xhci rather than NEC xhci to Q35 domains

qemu: auto-add generic xhci rather than NEC xhci to Q35 domains

We recently added a generic XHCI USB3 controller to QEMU, and libvirt
supports adding that controller rather than the NEC XHCI USB3
controller, but when auto-adding a USB controller to Q35 domains we
were still adding the vendor-specific NEC controller. This patch
changes to add the generic controller instead, if it's available in
the QEMU binary that will be used.

Signed-off-by: Laine Stump <laine@laine.org>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>