examples/vhost: add --dequeue-zero-copy option

examples/vhost: add --dequeue-zero-copy option

Add an option, --dequeue-zero-copy, to enable dequeue zero copy.

One thing worth noting while using dequeue zero copy is the nb_tx_desc
has to be small enough so that the eth driver will hit the mbuf free
threshold easily and thus free mbuf more frequently.

The reason behind that is, when dequeue zero copy is enabled, guest Tx
used vring will be updated only when corresponding mbuf is freed. If mbuf
is not freed frequently, the guest Tx vring could be starved.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Qian Xu <qian.q.xu@intel.com>