Restructure flake.nix for a simpler config for building different images off the same NixOS config.

nix/configuration/formats/vm_iso.nix

@@ -0,0 +1,21 @@
+{
+  modulesPath,
+  ...
+}:
+
+{
+  imports = [
+    (modulesPath + "/installer/cd-dvd/iso-image.nix")
+    (modulesPath + "/profiles/qemu-guest.nix") # VirtIO kernel modules
+  ];
+
+  config = {
+    isoImage.makeEfiBootable = true;
+    isoImage.makeUsbBootable = true;
+
+    networking.dhcpcd.enable = true;
+    networking.useDHCP = true;
+
+    me.image_based_appliance.enable = true;
+  };
+}