Tradeoff: poll or notify

Polling:

• Server cannot report size unless client asks
• Each server reply sets NBD_REPLY_DONE flag bit
• Client must ask in a loop when polling for a change from the server

Notify:

• Client uses NBD_CMD_FLAG_RESIZE_NOTIFY to request an open-ended command
• Server replies with NBD_REPLY_DONE flag clear whenever it has a new size to report