qemu: Don't append 0 at usb id, so that it is compatible with legacy -usb

qemu: Don't append 0 at usb id, so that it is compatible with legacy -usb

QEMU uses USB bus name "usb.0" when using the legacy -usb argument.
If we want to allow USB devices to specify their addresses with legacy
-usb, we should either in case of legacy bus name drop the 0 from the
address bus, or just drop the 0 from device id. This patch does the
later.

Another solution would be to permit addressing on non-legacy USB
controllers only.