This function combines bdrv_set_backing_hd() and bdrv_replace_node()
so we can use it to simplify the code a bit in commit_start().
Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 20190403143748.9790-1-berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>