Don't set an initial null root password for Amazon / VirtualBox images

A null password allows logging into local PAM services such as "login"
(agetty) and KDM.  That's not actually a security problem for EC2
machines, since they do not have "local" logins; for VirtualBox
machines, if you local access, you can do anything anyway.  But it's
better to be on the safe side and disable password-based logins for
root.

nixos/modules/virtualisation/virtualbox-image.nix

@@ -107,4 +107,9 @@ with pkgs.lib;
   boot.loader.grub.device = "/dev/sda";
 
   services.virtualbox.enable = true;
+
+  # Prevent logging in as root without a password.  For NixOps, we
+  # don't need this because the user can login via SSH, and for the
+  # demo images, there is a demo user account that can sudo to root.
+  security.initialRootPassword = "!";
 }