nixos/tests/virtualbox: Destroy detectvirt VM.

I forgot to do this in da0e642. It shouldn't be a big problem but it's
more clean to destroy the VM once we're done testing.

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

nixos/tests/virtualbox.nix

@@ -413,6 +413,7 @@ in {
       shutdownVM_detectvirt;
       my $result = $machine->succeed("cat '$detectvirt_sharepath/result'");
       chomp $result;
+      destroyVM_detectvirt;
       die "systemd-detect-virt returned \"$result\" instead of \"oracle\""
         if $result ne "oracle";
     };