VMDK: flush multiple extents

Flush all the file that referenced by the image.
Signed-off-by: NFam Zheng <famcool@gmail.com>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/vmdk.c

@@ -1072,7 +1072,17 @@ static void vmdk_close(BlockDriverState *bs)
 
 static int vmdk_flush(BlockDriverState *bs)
 {
-    return bdrv_flush(bs->file);
+    int i, ret, err;
+    BDRVVmdkState *s = bs->opaque;
+
+    ret = bdrv_flush(bs->file);
+    for (i = 0; i < s->num_extents; i++) {
+        err = bdrv_flush(s->extents[i].file);
+        if (err < 0) {
+            ret = err;
+        }
+    }
+    return ret;
 }