nixos/tests/udisks2: Fix USB device hotplug

The usb_add and usb_del monitor commands have been removed in QEMU
version 2.12 (introduced in 3e3b39f1).

Quote from https://wiki.qemu.org/ChangeLog/2.12#Incompatible_changes

:

> The deprecated HMP commands "usb_add" and "usb_del" have been removed.
> Use "device_add" and "device_del" as replacement instead.

So we're doing exactly that and the udisks2 test now works again.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @edolstra