Mechanically convert if_vtnet(4) to DrvAPI

[bryanv]

sys/dev/virtio/network/if_vtnet.c

164–165

Nit: looks like the prototypes with struct ifnet * as the last arg ended up with a trailing space after if_t

[jhibbits]

sys/dev/virtio/network/if_vtnet.c

164–165

This is probably the case in most drivers I changed, since the changes were purely mechanical (sed+awk). I'll do a cleanup pass before pushing them all.