virtualisation: Make parallels handling more lazy

[britter]

Moves the condition to allow parallels as an unfree module up one level.
This is required for users who don't instantiate their own nixpkgs
instance which causes options.nixpkgs.pkgs.isDefined being false.
In this case, although cfg.parallels.enable is false, the
allowUnfreePredicate will still be set resulting an error complaining
about allowUnfreePredicate option being used but not defined.

[britter]

Tried this on my configuration with a local checkout of this repository. Without this change, rebuilding my configuration will fail with

building the system configuration...
warning: Git tree '/home/bene/github/britter/nix-configuration' is dirty
error:
       … while calling the 'head' builtin

         at /nix/store/z7j461v8da67nwrad4xz50dx2l5wk0wz-source/lib/attrsets.nix:1575:11:

         1574|         || pred here (elemAt values 1) (head values) then
         1575|           head values
             |           ^
         1576|         else

       … while evaluating the attribute 'value'

         at /nix/store/z7j461v8da67nwrad4xz50dx2l5wk0wz-source/lib/modules.nix:809:9:

          808|     in warnDeprecation opt //
          809|       { value = builtins.addErrorContext "while evaluating the option `${showOption loc}':" value;
             |         ^
          810|         inherit (res.defsFinal') highestPrio;

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: The option `allowUnfreePredicate' is used but not defined.

After applying this fix to my local checkout, I can build my configuration again.