examples/vhost: fix retry logic on Rx path

examples/vhost: fix retry logic on Rx path

drain_eth_rx() uses rte_vhost_avail_entries() to calculate
the available entries to determine if a retry is required.
However, this function only works with split rings, and
calculating packed rings will return the wrong value and cause
unnecessary retries resulting in a significant performance penalty.

This patch fix that by using the difference between tx/rx burst
as the retry condition.

Fixes: be800696c26e ("examples/vhost: use burst enqueue and dequeue from lib")
Cc: stable@dpdk.org
Signed-off-by: Yuan Wang <yuanx.wang@intel.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Tested-by: Wei Ling <weix.ling@intel.com>