nixos: Add OCI image options

Follows what amazon images does.

nixos/modules/module-list.nix

···
   ./virtualisation/nixos-containers.nix
   ./virtualisation/oci-containers.nix
   ./virtualisation/openstack-options.nix
+  ./virtualisation/oci-options.nix
   ./virtualisation/openvswitch.nix
   ./virtualisation/parallels-guest.nix
   ./virtualisation/podman/default.nix

nixos/modules/virtualisation/oci-options.nix

···
+{ config, lib, pkgs, ... }:
+{
+  options = {
+    oci = {
+      efi = lib.mkOption {
+        default = pkgs.stdenv.hostPlatform.isAarch64;
+        internal = true;
+        description = ''
+          Whether the OCI instance is using EFI.
+        '';
+      };
+    };
+  };
+}