machine: Improve error message when using default RAM backend id

For migration purposes, users might want to reuse the default RAM
backend id, but specify a different memory backend.

For example, to reuse "pc.ram" on q35, one has to set
    -machine q35,memory-backend=pc.ram
Only then, can a memory backend with the id "pc.ram" be created
manually.

Let's improve the error message by improving the hint. Use
error_append_hint() -- which in turn requires ERRP_GUARD().

Message-ID: <20230906120503.359863-12-david@redhat.com>
Suggested-by: ThinerLogoer <logoerthiner1@163.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

hw/core/machine.c

@@ -1355,6 +1355,7 @@ static bool create_default_memdev(MachineState *ms, const char *path, Error **er
 
 void machine_run_board_init(MachineState *machine, const char *mem_path, Error **errp)
 {
+    ERRP_GUARD();
     MachineClass *machine_class = MACHINE_GET_CLASS(machine);
     ObjectClass *oc = object_class_by_name(machine->cpu_type);
     CPUClass *cc;
@@ -1383,9 +1384,13 @@ void machine_run_board_init(MachineState *machine, const char *mem_path, Error *
                numa_uses_legacy_mem()) {
         if (object_property_find(object_get_objects_root(),
                                  machine_class->default_ram_id)) {
-            error_setg(errp, "object name '%s' is reserved for the default"
-                " RAM backend, it can't be used for any other purposes."
-                " Change the object's 'id' to something else",
+            error_setg(errp, "object's id '%s' is reserved for the default"
+                " RAM backend, it can't be used for any other purposes",
+                machine_class->default_ram_id);
+            error_append_hint(errp,
+                "Change the object's 'id' to something else or disable"
+                " automatic creation of the default RAM backend by setting"
+                " 'memory-backend=%s' with '-machine'.\n",
                 machine_class->default_ram_id);
             return;
         }