cpu: Document why cannot_instantiate_with_device_add_yet

Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

qom/cpu.c

@@ -254,7 +254,11 @@ static void cpu_class_init(ObjectClass *klass, void *data)
     k->gdb_read_register = cpu_common_gdb_read_register;
     k->gdb_write_register = cpu_common_gdb_write_register;
     dc->realize = cpu_common_realizefn;
-    dc->cannot_instantiate_with_device_add_yet = true; /* FIXME explain why */
+    /*
+     * Reason: CPUs still need special care by board code: wiring up
+     * IRQs, adding reset handlers, halting non-first CPUs, ...
+     */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo cpu_type_info = {