net/vhost: do not count unsent packets as errors

net/vhost: do not count unsent packets as errors

missed_pkts reflects the number of packets that the driver did not manage
to send.
This is a temporary situation, those packets are not freed and the
application can still retry to send them later.
Hence, we can't count them as transmit failed.

Fixes: 5f05e95cd5d9 ("net/vhost: fix Tx error counting")
Fixes: ee584e9710b9 ("vhost: add driver on top of the library")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>