libvirtd: add parallelShutdown option

This adds a new ``parallelShutdown`` option that allows users to control
how many guests can be shut down concurrently. Allowing multiple virtual
machines to be shut down at the same time reduces the amount of time it
takes to reboot the host.

Upstream documentation: https://www.libvirt.org/manpages/libvirt-guests.html#files

nixos/modules/virtualisation/libvirtd.nix

@@ -220,6 +220,17 @@ in
       '';
     };
 
+    parallelShutdown = mkOption {
+      type = types.ints.unsigned;
+      default = 0;
+      description = lib.mdDoc ''
+        Number of guests that will be shutdown concurrently, taking effect when onShutdown
+        is set to "shutdown". If set to 0, guests will be shutdown one after another.
+        Number of guests on shutdown at any time will not exceed number set in this
+        variable.
+      '';
+    };
+
     allowedBridges = mkOption {
       type = types.listOf types.str;
       default = [ "virbr0" ];
@@ -373,6 +384,7 @@ in
 
       environment.ON_BOOT = "${cfg.onBoot}";
       environment.ON_SHUTDOWN = "${cfg.onShutdown}";
+      environment.PARALLEL_SHUTDOWN = "${toString cfg.parallelShutdown}";
     };
 
     systemd.sockets.virtlogd = {

pkgs/development/libraries/libvirt/default.nix

@@ -341,6 +341,7 @@ stdenv.mkDerivation rec {
     substituteInPlace $out/libexec/libvirt-guests.sh \
       --replace 'ON_BOOT="start"'       'ON_BOOT=''${ON_BOOT:-start}' \
       --replace 'ON_SHUTDOWN="suspend"' 'ON_SHUTDOWN=''${ON_SHUTDOWN:-suspend}' \
+      --replace 'PARALLEL_SHUTDOWN=0'   'PARALLEL_SHUTDOWN=''${PARALLEL_SHUTDOWN:-0}' \
       --replace "$out/bin"              '${gettext}/bin' \
       --replace 'lock/subsys'           'lock' \
       --replace 'gettext.sh'            'gettext.sh