libvirtd-service: give access to users in the "libvirtd" group

Currently only root has access. But with this patch all users in
"libvirtd" group will have access. This is similar to how it's done on
Ubuntu.

Also, add virtualisation.libvirtd.extraConfig option for further
customization of libvirtd.conf.

modules/virtualisation/libvirtd.nix

@@ -7,6 +7,13 @@ with pkgs.lib;
 let
 
   cfg = config.virtualisation.libvirtd;
+  configFile = pkgs.writeText "libvirtd.conf" ''
+    unix_sock_group = "libvirtd"
+    unix_sock_rw_perms = "0770"
+    auth_unix_ro = "none"
+    auth_unix_rw = "none"
+    ${cfg.extraConfig}
+  '';
 
 in
 
@@ -36,6 +43,16 @@ in
           '';
       };
 
+    virtualisation.libvirtd.extraConfig =
+      mkOption {
+        default = "";
+        description =
+          ''
+            Extra contents appended to the libvirtd configuration file,
+            libvirtd.conf.
+          '';
+      };
+
   };
 
 
@@ -83,7 +100,7 @@ in
             done
           ''; # */
 
-        serviceConfig.ExecStart = "@${pkgs.libvirt}/sbin/libvirtd libvirtd --daemon --verbose";
+        serviceConfig.ExecStart = ''@${pkgs.libvirt}/sbin/libvirtd libvirtd --config "${configFile}" --daemon --verbose'';
         serviceConfig.Type = "forking";
         serviceConfig.KillMode = "process"; # when stopping, leave the VMs alone
 
@@ -124,6 +141,8 @@ in
         serviceConfig.RemainAfterExit = true;
       };
 
+    users.extraGroups.libvirtd = {};
+
   };
 
 }