migration/block-dirty-bitmap: fix dirty_bitmap_load

dirty_bitmap_load_header return code is obtained but not handled. Fix
this.

Bug was introduced in b35ebdf
"migration: add postcopy migration of dirty bitmaps" with the whole
function.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180530112424.204835-1-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/block-dirty-bitmap.c

@@ -672,6 +672,9 @@ static int dirty_bitmap_load(QEMUFile *f, void *opaque, int version_id)
 
     do {
         ret = dirty_bitmap_load_header(f, &s);
+        if (ret < 0) {
+            return ret;
+        }
 
         if (s.flags & DIRTY_BITMAP_MIG_FLAG_START) {
             ret = dirty_bitmap_load_start(f, &s);