qdev: Implement qdev_create_fake_machine() for user emulation

When a QDev instance is realized, qdev_get_machine() ends up called.
In the next commit, qdev_get_machine() will require a "machine"
container to be always present. To satisfy this QOM containers design,
Implement qdev_create_fake_machine() which creates a fake "machine"
container for user emulation.

On system emulation, qemu_create_machine() is called from qemu_init().
For user emulation, since the TCG accelerator always calls
tcg_init_machine(), we use it to hook our fake machine creation.

Suggested-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250102211800.79235-2-philmd@linaro.org>

hw/core/qdev-user.c

@@ -0,0 +1,19 @@
+/*
+ * QDev helpers specific to user emulation.
+ *
+ * Copyright 2025 Linaro, Ltd.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+#include "qemu/osdep.h"
+#include "qom/object.h"
+#include "hw/qdev-core.h"
+
+void qdev_create_fake_machine(void)
+{
+    Object *fake_machine_obj;
+
+    fake_machine_obj = object_property_add_new_container(object_get_root(),
+                                                         "machine");
+    object_property_add_new_container(fake_machine_obj, "unattached");
+}