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>

include/ui/dbus-display.h

@@ -0,0 +1,17 @@
+#ifndef DBUS_DISPLAY_H_
+#define DBUS_DISPLAY_H_
+
+#include "qapi/error.h"
+#include "ui/dbus-module.h"
+
+static inline bool qemu_using_dbus_display(Error **errp)
+{
+    if (!using_dbus_display) {
+        error_set(errp, ERROR_CLASS_DEVICE_NOT_ACTIVE,
+                  "D-Bus display is not in use");
+        return false;
+    }
+    return true;
+}
+
+#endif /* DBUS_DISPLAY_H_ */

include/ui/dbus-module.h

@@ -0,0 +1,11 @@
+#ifndef DBUS_MODULE_H_
+#define DBUS_MODULE_H_
+
+struct QemuDBusDisplayOps {
+    bool (*add_client)(int csock, Error **errp);
+};
+
+extern int using_dbus_display;
+extern struct QemuDBusDisplayOps qemu_dbus_display;
+
+#endif /* DBUS_MODULE_H_*/