ui/dbus: add p2p=on/off option

Add an option to use direct connections instead of via the bus. Clients
are accepted with QMP add_client.

This allows to provide the D-Bus display without a bus. It also
simplifies the testing setup (some CI have issues to setup a D-Bus bus
in a container).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>

ui/dbus-module.c

@@ -0,0 +1,35 @@
+/*
+ * D-Bus module support.
+ *
+ * Copyright (C) 2021 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 or
+ * (at your option) version 3 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "qemu/osdep.h"
+#include "qapi/error.h"
+#include "ui/dbus-module.h"
+
+int using_dbus_display;
+
+static bool
+qemu_dbus_display_add_client(int csock, Error **errp)
+{
+    error_setg(errp, "D-Bus display isn't enabled");
+    return false;
+}
+
+struct QemuDBusDisplayOps qemu_dbus_display = {
+    .add_client = qemu_dbus_display_add_client,
+};