fruity: Improve TcpTunnelConnection teardown

So the VirtualNetworkStack is torn down right away upon the remote end
closing the connection, instead of once close() is called.

Also ensure any pending write operation is cancelled at that point.

src/fruity/xpc.vala

@@ -1979,9 +1979,6 @@ namespace Frida.Fruity {
 			} catch (Error e) {
 				assert_not_reached ();
 			}
-
-			if (_tunnel_netstack != null)
-				_tunnel_netstack.stop ();
 		}
 
 		private async void process_incoming_messages () {
@@ -1994,6 +1991,8 @@ namespace Frida.Fruity {
 			} catch (GLib.Error e) {
 			}
 
+			io_cancellable.cancel ();
+
 			var source = new IdleSource ();
 			source.set_callback (process_incoming_messages.callback);
 			source.attach (MainContext.get_thread_default ());
@@ -2004,6 +2003,9 @@ namespace Frida.Fruity {
 			} catch (GLib.Error e) {
 			}
 
+			if (_tunnel_netstack != null)
+				_tunnel_netstack.stop ();
+
 			close_request.resolve (true);
 
 			closed ();