qemu: require big-parallel

Compiles in about 2h50m on a 2-core builder, and 20m on a big-parallel
machine.

pkgs/applications/virtualization/qemu/default.nix

@@ -177,6 +177,9 @@ stdenv.mkDerivation rec {
     qemu-system-i386 = "bin/qemu-system-i386";
   };
 
+  # Builds in ~3h with 2 cores, and ~20m with a big-parallel builder.
+  requiredSystemFeatures = [ "big-parallel" ];
+
   meta = with lib; {
     homepage = "http://www.qemu.org/";
     description = "A generic and open source machine emulator and virtualizer";