nbd_alloc_request
status_t nbd_alloc_request(struct nbd_device *dev, struct nbd_request_entry **req, uint32 type, off_t from, size_t len, const char *data);
err = nbd_alloc_request(dev, &req, NBD_CMD_READ, position, *numbytes, NULL);
err = nbd_alloc_request(dev, &req, NBD_CMD_WRITE, position, *numbytes, data);