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>