allow to use multiple cores in make-disk-image

Author: Mic92

lib/make-disk-image.nix

@@ -215,6 +215,11 @@ in
     build_memory=''${build_memory:-${builtins.toString diskoCfg.memSize}}
     QEMU_OPTS=${lib.escapeShellArg QEMU_OPTS}
     QEMU_OPTS+=" -m $build_memory"
+    if [ ''${NIX_BUILD_CORES:-0} = 0 ]; then
+      QEMU_OPTS+=" -smp cpus=$(nproc)"
+    else
+      QEMU_OPTS+=" -smp cpus=$NIX_BUILD_CORES"
+    fi
     export QEMU_OPTS
 
     ${pkgs.bash}/bin/sh -e ${vmTools.vmRunCommand vmTools.qemuCommandLinux}