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.

Changed files
+2 -1
nixos
lib
test-driver
+2 -1
nixos/lib/test-driver/test-driver.py
···
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 "