Also add a nicer error message when the firmware package is not
installed.

It is only useful when debugging and otherwise just throws a whole bunch
of logs on every boot.

fallocate immediately allocates the full size of the file, even though
it likely won't end up being used.

hda-duplex allows recording too. At least for XFCE this results in a
scary mic recording icon even when nothing inside the VM is actually
recording anything.