usbtest: Fix issue when multiple devices are sharing same USB vendor and product ID.

When there are multiple devices sharing the same USB vendor and product ID,
the wrong device may be selected. Fix this by also matching the bus and
device address, ugen<X>.<Y> .

Sponsored by: NVIDIA Networking
Approved by: re (gjb)

(cherry picked from commit 16346e1401b8b369e251bc70781349fb9b813cef)
(cherry picked from commit 90afe1886f5250dc32134b5851adf6cffa4a46d4)