nixos/test-driver: use usb-ehci controller instead of piix3-usb-uhci

On my system this change offers ~5X speed up of
nixosTests.boot.biosUsb and nixosTests.boot.uefiUsb tests.

nixos/lib/test-driver/test-driver.py

@@ -307,8 +307,9 @@ class Machine:
             start_command += "-cdrom " + args["cdrom"] + " "
 
         if "usb" in args:
+            # https://github.com/qemu/qemu/blob/master/docs/usb2.txt
             start_command += (
-                "-device piix3-usb-uhci -drive "
+                "-device usb-ehci -drive "
                 + "id=usbdisk,file="
                 + args["usb"]
                 + ",if=none,readonly "