mkosi: add qemu for starting vms

Adding the option to build mkosi with qemu support, which enables
the usage of 'mkosi qemu' to boot the previously built image as VM.
This will only work for host=target right now, further effort
is required to allow target with different arch.

Co-authored-by: Malte Poll <mp@edgeless.systems>
Signed-off-by: Paul Meyer <49727155+katexochen@users.noreply.github.com>

pkgs/tools/virtualization/mkosi/default.nix

@@ -1,15 +1,22 @@
 { lib
 , fetchFromGitHub
+, stdenv
 , python3
+, bubblewrap
+, systemd
+
+  # Python packages
 , setuptools
 , setuptools-scm
 , wheel
 , buildPythonApplication
 , pytestCheckHook
 , pefile
-, bubblewrap
-, systemd
-, stdenv
+
+  # Optional dependencies
+, withQemu ? false
+, qemu
+, OVMF
 }:
 let
   # For systemd features used by mkosi, see
@@ -43,10 +50,14 @@ buildPythonApplication rec {
   # Fix ctypes finding library
   # https://github.com/NixOS/nixpkgs/issues/7307
   patchPhase = lib.optionalString stdenv.isLinux ''
-    substituteInPlace mkosi/run.py --replace \
-      'ctypes.util.find_library("c")' "'${stdenv.cc.libc}/lib/libc.so.6'"
-    substituteInPlace mkosi/__init__.py --replace \
-      '/usr/lib/systemd/ukify' "${systemdForMkosi}/lib/systemd/ukify"
+    substituteInPlace mkosi/run.py \
+      --replace 'ctypes.util.find_library("c")' "'${stdenv.cc.libc}/lib/libc.so.6'"
+    substituteInPlace mkosi/__init__.py \
+      --replace '/usr/lib/systemd/ukify' "${systemdForMkosi}/lib/systemd/ukify"
+  '' + lib.optionalString withQemu ''
+    substituteInPlace mkosi/qemu.py \
+      --replace '/usr/share/ovmf/x64/OVMF_VARS.fd' "${OVMF.variables}" \
+      --replace '/usr/share/ovmf/x64/OVMF_CODE.fd' "${OVMF.firmware}"
   '';
 
   nativeBuildInputs = [
@@ -62,6 +73,8 @@ buildPythonApplication rec {
   propagatedBuildInputs = [
     systemdForMkosi
     bubblewrap
+  ] ++ lib.optional withQemu [
+    qemu
   ];
 
   postInstall = ''