Comment 1 for bug 1933610

    while in one host，if creating the instance and binding port failed，it will raise the PortBindingailed exception; And in the _build_resource function，will catch the exception and call the _shutdown_instance. In the _shutdown_instance fucntion， it will call the self.volume_api.attachment_delete(context, bdm.attachment_id) to delete the attachment_id， because we have created the volume and attachment_id in the _build_resource fucntion.
    If we schedule the instance to another host, we have the volume_id and will not recreate the attachment_id again for the volume, However, the attachment_id of the volume has been deleted in the first host. So, we should optimize the code in block_device.py.