nixos/tests/vbox: Disable audio for VBox guests

We don't have (simulated) sound hardware within the qemu VM, neither do
we have it available within VirtualBox that's running within the qemu
VMs.

With sound hardware the VirtualBox UI displays an error dialog, which in
turn causes the VM process to hang on unregister. This in turn has
caused the tests to fail because of the following error:

Cannot unregister the machine '...' while it is locked

Signed-off-by: aszlig <aszlig@redmoonstudios.org>