Currently, only ``RTE_VHOST_ASYNC_INORDER`` capable device is
supported by vhost.
- * ``async_threshold``
-
- The copy length (in bytes) below which CPU copy will be used even if
- applications call async vhost APIs to enqueue/dequeue data.
-
- Typical value is 256~1024 depending on the async device capability.
-
Applications must provide following ``ops`` callbacks for vhost lib to
work with the async copy devices: