Draft: Trim down the QEMU build

Refs #88

Trying to slim down our deployed software.

This MR builds on a suggestion by @jcalderone in #88. I don't think I am doing it right though, since the reported difference is minimal:

$ nix --extra-experimental-features nix-command store diff-closures result-before/ result-after/

initrd-linux: +13.1 KiB

morph/lib/default.nix

@@ -33,7 +33,12 @@
     # the package set only needs to be evaluted once for the grid, rather
     # than once for each host.
     overlays = [
-      (self: super: { ourpkgs = self.callPackage ../../nixos/pkgs {}; })
+      (self: super: {
+        ourpkgs = self.callPackage ../../nixos/pkgs {};
+        qemu = super.qemu.override {
+          nixosTestRunner = true; # turns off GUI and audio
+        };
+      })
     ];
   };
 }