Fix a couple of silly mistakes.
- Fix typo: nmd->cur_tx_ring should be used in pci_vtnet_netmap_writev()
- The buffer length should be checked to avoid overflow, but there is no API to get the slot length, so the hardcoded value is used.
- Return the currently-first request chain back to the available queue if there is no more packets.
- Report the link as up if we managed to open vale port.
- Use consistent coding style.
This is the worst patch I have ever submitted. I'm very guilty for
that, and I promise it won't happen again.