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[1] is the worst patch I have ever submitted. I'm very guilty for
that, and I promise it won't happen again.

[1] https://lists.freebsd.org/pipermail/freebsd-virtualization/2015-January/003350.html