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 are no more packets.
Report the link as up if we managed to open vale port.
Use consistent coding style.

Submitted by: btw
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D5595