xen, qemu: passthru the path to qemu-system-i386

pkgs/applications/virtualization/qemu/default.nix

···
     else if stdenv.isAarch64 then ''makeWrapper $out/bin/qemu-system-aarch64 $out/bin/qemu-kvm --add-flags "\$([ -e /dev/kvm ] && echo -enable-kvm)"''
     else "";
 
+  passthru = {
+    qemu-system-i386 = "bin/qemu-system-i386";
+  };
+
   meta = with stdenv.lib; {
     homepage = http://www.qemu.org/;
     description = "A generic and open source machine emulator and virtualizer";

pkgs/applications/virtualization/xen/4.5.nix

···
       -i tools/libxl/libxl_device.c
   '';
 
+  passthru = {
+    qemu-system-i386 = if withInternalQemu
+      then "lib/xen/bin/qemu-system-i386"
+      else throw "this xen has no qemu builtin";
+  };
+
 })) ({ ocamlPackages = ocamlPackages_4_02; } // args)

pkgs/applications/virtualization/xen/4.8.nix

···
       -i tools/libxl/libxl_device.c
   '';
 
+  passthru = {
+    qemu-system-i386 = if withInternalQemu
+      then "lib/xen/bin/qemu-system-i386"
+      else throw "this xen has no qemu builtin";
+  };
+
 })) args