vmxnet3: fix stop/restart
vmxnet3: fix stop/restart

This change makes vmxnet3 consistent with other pmds in
terms of dev_stop behavior: rather than releasing tx/rx
rings, it only resets the ring structure and release the
pending mbufs.

Verified with various tests (test-pmd and pktgen) over
vmxnet3 that dev stop/restart works fine.

Signed-off-by: Yong Wang <0cd1be787884a033c98e298059e386114d22793e@vmware.com>

phermansson/dpdk,phermansson/dpdk,phermansson/dpdk,phermansson/dpdk