machine: do not crash if default RAM backend name has been stolen

QEMU aborts when default RAM backend should be used (i.e. no
explicit '-machine memory-backend=' specified) but user
has created an object which 'id' equals to default RAM backend
name used by board.

 $QEMU -machine pc \
       -object memory-backend-ram,id=pc.ram,size=4294967296

 Actual results:
 QEMU 7.2.0 monitor - type 'help' for more information
 (qemu) Unexpected error in object_property_try_add() at ../qom/object.c:1239:
 qemu-kvm: attempt to add duplicate property 'pc.ram' to object (type 'container')
 Aborted (core dumped)

Instead of abort, check for the conflicting 'id' and exit with
an error, suggesting how to remedy the issue.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2207886
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20230522131717.3780533-1-imammedo@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit a37531f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/core/machine.c

@@ -1329,6 +1329,14 @@ void machine_run_board_init(MachineState *machine, const char *mem_path, Error *
         }
     } else if (machine_class->default_ram_id && machine->ram_size &&
                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",
+                machine_class->default_ram_id);
+            return;
+        }
         if (!create_default_memdev(current_machine, mem_path, errp)) {
             return;
         }