There are per-machine type variations on which usb controller will be
picked on an x86_64 machine. Add test cases where a USB controller is
completely missing to cover all 3 cases ('isapc', 'pc', 'q35') when an
USB controller is not explicitly requested by the user.
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
