kernel: set virtualization options regardless of grsec

Per my own testing, the NixOS grsecurity kernel works both as a
KVM-based virtualisation host and guest; there appears to be no good
reason to making these conditional on `features.grsecurity`.

More generally, it's unclear what `features.grsecurity` *means*. If
someone configures a grsecurity kernel in such a fashion that it breaks
KVM support, they should know to disable KVM themselves.