qcow2: flush refcount cache correctly in qcow2_write_snapshots()

Since qcow2 metadata is cached we need to flush the caches, not just the
underlying file.  Use bdrv_flush(bs) instead of bdrv_flush(bs->file).

Also add the error return path when bdrv_flush() fails and move the
flush after checking for qcow2_alloc_clusters() failure so that the
qcow2_alloc_clusters() error return value takes precedence.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit f6977f15)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

block/qcow2-snapshot.c

@@ -180,11 +180,14 @@ static int qcow2_write_snapshots(BlockDriverState *bs)
 
     /* Allocate space for the new snapshot list */
     snapshots_offset = qcow2_alloc_clusters(bs, snapshots_size);
-    bdrv_flush(bs->file);
     offset = snapshots_offset;
     if (offset < 0) {
         return offset;
     }
+    ret = bdrv_flush(bs);
+    if (ret < 0) {
+        return ret;
+    }
 
     /* Write all snapshots to the new list */
     for(i = 0; i < s->nb_snapshots; i++) {