acpi, piix4: Add unplug cb for piix4.

Memory and CPU hot unplug are both asynchronous procedures.
When the unplug operation happens, unplug request cb is called first.
And when guest OS finished handling unplug, unplug cb will be called
to do the real removal of device.

This patch adds hotunplug cb to piix4, which memory and CPU
hot unplug will use it.
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>
Signed-off-by: NTang Chen <tangchen@cn.fujitsu.com>
Signed-off-by: NZhu Guihua <zhugh.fnst@cn.fujitsu.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/acpi/piix4.c

@@ -370,6 +370,13 @@ static void piix4_device_unplug_request_cb(HotplugHandler *hotplug_dev,
     }
 }
 
+static void piix4_device_unplug_cb(HotplugHandler *hotplug_dev,
+                                   DeviceState *dev, Error **errp)
+{
+    error_setg(errp, "acpi: device unplug for not supported device"
+               " type: %s", object_get_typename(OBJECT(dev)));
+}
+
 static void piix4_update_bus_hotplug(PCIBus *pci_bus, void *opaque)
 {
     PIIX4PMState *s = opaque;
@@ -609,6 +616,7 @@ static void piix4_pm_class_init(ObjectClass *klass, void *data)
     dc->hotpluggable = false;
     hc->plug = piix4_device_plug_cb;
     hc->unplug_request = piix4_device_unplug_request_cb;
+    hc->unplug = piix4_device_unplug_cb;
     adevc->ospm_status = piix4_ospm_status;
 }