nixos/tests/vbox: Fix invocation of dbus

VBoxService needs dbus in order to work properly, which failed to start
up so far, because it was searching in /run/current-system/sw for its
configuration files.

We now no longer run with the --system flag but specify the
configuration file directly instead.

This fixes at least the "simple-gui" test and probably the others as
well, which I haven't tested yet.

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