libvirtd: don't start libvirtd-tcp.socket by default

Per upstream:

> libvirtd-tcp.socket - the unit file corresponding to the TCP 16509
> port for non-TLS remote access. This socket should not be configured
> to start on boot until the administrator has configured a suitable
> authentication mechanism.

(cherry picked from commit 84ecbc9)

nixos/modules/virtualisation/libvirtd.nix

@@ -267,7 +267,7 @@ in {
       restartIfChanged = false;
     };
 
-    systemd.sockets.libvirtd    .wantedBy = [ "sockets.target" ];
-    systemd.sockets.libvirtd-tcp.wantedBy = [ "sockets.target" ];
+    # https://libvirt.org/daemons.html#monolithic-systemd-integration
+    systemd.sockets.libvirtd.wantedBy = [ "sockets.target" ];
   };
 }