netdev-dpdk: Remove vhost send retries when no packets have been sent.

netdev-dpdk: Remove vhost send retries when no packets have been sent.

If the guest is connected but not servicing the virt queue, this leads
to vhost send retries until timeout. This is fine in isolation but if
there are other high rate queues also being serviced by the same PMD
it can lead to a performance hit on those queues. Change to only retry
when at least some packets have been successfully sent on the previous
attempt.

Also, limit retries to avoid a similar delays if packets are being sent
at a very low rate due to few available descriptors.

Reported-by: Bhanuprakash Bodireddy <bhanuprakash.bodireddy@intel.com>
Signed-off-by: Kevin Traynor <kevin.traynor@intel.com>
Acked-by: Bhanuprakash Bodireddy <bhanuprakash.bodireddy@intel.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>