tests/virtualbox: Use antiquotes for log sockets.

We're simply using antiquotation, since it's been a while since these
got introduced (in Nix 1.7). So we can use them because it makes the
code much more readable.

As usual, I made sure that I didn't accidentally change something in
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>