block: Convert bdrv_co_readv() to BdrvChild

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/block/qcow.c b/block/qcow.c
index e4175b8..c8d4e5f 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -619,8 +619,7 @@
                 hd_iov.iov_len = n * 512;
                 qemu_iovec_init_external(&hd_qiov, &hd_iov, 1);
                 qemu_co_mutex_unlock(&s->lock);
-                ret = bdrv_co_readv(bs->backing->bs, sector_num,
-                                    n, &hd_qiov);
+                ret = bdrv_co_readv(bs->backing, sector_num, n, &hd_qiov);
                 qemu_co_mutex_lock(&s->lock);
                 if (ret < 0) {
                     goto fail;
@@ -644,7 +643,7 @@
             hd_iov.iov_len = n * 512;
             qemu_iovec_init_external(&hd_qiov, &hd_iov, 1);
             qemu_co_mutex_unlock(&s->lock);
-            ret = bdrv_co_readv(bs->file->bs,
+            ret = bdrv_co_readv(bs->file,
                                 (cluster_offset >> 9) + index_in_cluster,
                                 n, &hd_qiov);
             qemu_co_mutex_lock(&s->lock);