hw/acpi: Do not create memory hotplug method when handler is not defined

With Hardware-reduced ACPI, the GED device will manage ACPI
hotplug entirely. As a consequence, make the memory specific
events AML generation optional. The code will only be added
when the method name is not NULL.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20190918130633.4872-3-shameerali.kolothum.thodi@huawei.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/acpi/memory_hotplug.c

@@ -712,10 +712,12 @@ void build_memory_hotplug_aml(Aml *table, uint32_t nr_mem,
     }
     aml_append(table, dev_container);
 
-    method = aml_method(event_handler_method, 0, AML_NOTSERIALIZED);
-    aml_append(method,
-        aml_call0(MEMORY_DEVICES_CONTAINER "." MEMORY_SLOT_SCAN_METHOD));
-    aml_append(table, method);
+    if (event_handler_method) {
+        method = aml_method(event_handler_method, 0, AML_NOTSERIALIZED);
+        aml_append(method, aml_call0(MEMORY_DEVICES_CONTAINER "."
+                                     MEMORY_SLOT_SCAN_METHOD));
+        aml_append(table, method);
+    }
 
     g_free(mhp_res_path);
 }