migration: Block migration comment or code is wrong

And it appears that what is wrong is the code. During bulk stage we
need to make sure that some block is dirty, but no games with
max_size at all.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

migration/block.c

@@ -880,8 +880,8 @@ static void block_save_pending(QEMUFile *f, void *opaque, uint64_t max_size,
     blk_mig_unlock();
 
     /* Report at least one block pending during bulk phase */
-    if (pending <= max_size && !block_mig_state.bulk_completed) {
-        pending = max_size + BLK_MIG_BLOCK_SIZE;
+    if (!pending && !block_mig_state.bulk_completed) {
+        pending = BLK_MIG_BLOCK_SIZE;
     }
 
     trace_migration_block_save_pending(pending);