migration: Hack to maintain backwards compatibility for ppc

Current code does:
- register pre_2_10_vmstate_dummy_icp with "icp/server" and instance
  dependinfg on cpu number
- for newer machines, it register vmstate_icp with "icp/server" name
  and instance 0
- now it unregisters "icp/server" for the 1st instance.

This is wrong at many levels:
- we shouldn't have two VMSTATEDescriptions with the same name
- In case this is the only solution that we can came with, it needs to
  be:
  * register pre_2_10_vmstate_dummy_icp
  * unregister pre_2_10_vmstate_dummy_icp
  * register real vmstate_icp

Created vmstate_replace_hack_for_ppc() with warnings left and right
that it is a hack.

CC: Cedric Le Goater <clg@kaod.org>
CC: Daniel Henrique Barboza <danielhb413@gmail.com>
CC: David Gibson <david@gibson.dropbear.id.au>
CC: Greg Kurz <groug@kaod.org>

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231020090731.28701-8-quintela@redhat.com>

migration/savevm.c

@@ -846,6 +846,24 @@ static void vmstate_check(const VMStateDescription *vmsd)
     }
 }
 
+/*
+ * See comment in hw/intc/xics.c:icp_realize()
+ *
+ * This function can be removed when
+ * pre_2_10_vmstate_register_dummy_icp() is removed.
+ */
+int vmstate_replace_hack_for_ppc(VMStateIf *obj, int instance_id,
+                                 const VMStateDescription *vmsd,
+                                 void *opaque)
+{
+    SaveStateEntry *se = find_se(vmsd->name, instance_id);
+
+    if (se) {
+        savevm_state_handler_remove(se);
+    }
+    return vmstate_register(obj, instance_id, vmsd, opaque);
+}
+
 int vmstate_register_with_alias_id(VMStateIf *obj, uint32_t instance_id,
                                    const VMStateDescription *vmsd,
                                    void *opaque, int alias_id,