net/netvsc: reset mbuf port on VF Rx

Redo the receive logic to set m->port on packets received on VF.

When using VF, still need to check for packets and completions
arriving on the VMBus path even if application is not doing bursting
(ie n_rx == 0).

Also, fix comment.

Fixes: dc7680e8597c ("net/netvsc: support integrated VF")
Cc: stable@dpdk.org
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>