Set the kernel.poweroff_cmd sysctl

This allows Xen (and EC2) to power off an instance properly.  We had
this before (see aeb89fc753), but it got
lost in the systemd migration.

modules/system/boot/shutdown.nix

@@ -22,4 +22,6 @@ with pkgs.lib;
         '';
     };
 
+  boot.kernel.sysctl."kernel.poweroff_cmd" = "${config.systemd.package}/sbin/poweroff";
+
 }

tests/misc.nix

@@ -33,6 +33,11 @@
           $machine->waitForUnit("root-swapfile.swap");
           $machine->succeed("ls -l /root/swapfile | grep 134217728");
       };
+
+      # Test whether kernel.poweroff_cmd is set.
+      subtest "poweroff_cmd", sub {
+          $machine->succeed("[ -x \"\$(cat /proc/sys/kernel/poweroff_cmd)\" ]")
+      };
     '';
 
 }