numa: Make all callbacks of ram block notifiers optional

Let's make add/remove optional. We want to introduce a RAM block
notifier for RAM migration that is only interested in resize events.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210429112708.12291-4-david@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

hw/core/numa.c

@@ -818,8 +818,11 @@ static int ram_block_notify_add_single(RAMBlock *rb, void *opaque)
 void ram_block_notifier_add(RAMBlockNotifier *n)
 {
     QLIST_INSERT_HEAD(&ram_list.ramblock_notifiers, n, next);
+
     /* Notify about all existing ram blocks. */
-    qemu_ram_foreach_block(ram_block_notify_add_single, n);
+    if (n->ram_block_added) {
+        qemu_ram_foreach_block(ram_block_notify_add_single, n);
+    }
 }
 
 void ram_block_notifier_remove(RAMBlockNotifier *n)
@@ -832,7 +835,9 @@ void ram_block_notify_add(void *host, size_t size, size_t max_size)
     RAMBlockNotifier *notifier;
 
     QLIST_FOREACH(notifier, &ram_list.ramblock_notifiers, next) {
-        notifier->ram_block_added(notifier, host, size, max_size);
+        if (notifier->ram_block_added) {
+            notifier->ram_block_added(notifier, host, size, max_size);
+        }
     }
 }
 
@@ -841,7 +846,9 @@ void ram_block_notify_remove(void *host, size_t size, size_t max_size)
     RAMBlockNotifier *notifier;
 
     QLIST_FOREACH(notifier, &ram_list.ramblock_notifiers, next) {
-        notifier->ram_block_removed(notifier, host, size, max_size);
+        if (notifier->ram_block_removed) {
+            notifier->ram_block_removed(notifier, host, size, max_size);
+        }
     }
 }