nixos: Add VM test for VirtualBox.

Currently it pretty much tests starting up virtual machines and just
shutting down afterwards, but for both VBoxManage and the VirtualBox
GUI.

This helps catching errors in hardened mode, however we still need to
test whether networking works the way intended (and I fear that this is
broken at the moment).

The VirtualBox VM is _not_ using hardware virtualization support (thus
we use system = "i686-linux", because x86_64 has no emulation support),
because we're already within a qemu VM, which means it's going to be
slow as hell (that's why I've written own subs just for testing
startup/shutdown/whatnot with respective timeouts).

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