tests/virtualbox: Fix long line in guestAdditions.

This is essentially not only "wrapping" the line but refactoring into a
shorter name which is used in two places.

And yes, I know I'm very pedantic if it comes to whitespaces and line
lengths, but I made sure this doesn't change any functionality:

$ nix-instantiate nixos/tests/virtualbox.nix
...
/nix/store/cldxyrxqvwpqm02cd3lvknnmj4qmblyn-vm-test-run-virtualbox.drv
$ git stash pop
...
$ nix-instantiate nixos/tests/virtualbox.nix
...
/nix/store/cldxyrxqvwpqm02cd3lvknnmj4qmblyn-vm-test-run-virtualbox.drv
$

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