virttest.qemu_devices: Set port of the last usb device

When last device should be appended to usb bus, qemu automatically
add usb-hub to keep free ports for additional devices. We don't want
this in autotest as all tests use strict amount of usb buses. By
enforcing the last item's port we avoid this behavior.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>