nixos/tests/vbox: Add destroyVM for all subtests

One reason why it took me so long for debugging the test failure with
systemd-detect-virt was that simple-cli has succeeded while the former
has not.

This now makes sure we have consistency accross all the subtests and if
problems like the one in the previos commit ever show up again, we will
have just the headless test succeeding and it's more obvious where the
actual problem resides.

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