set correct tap device when NICTYPE is tap

- set TAPDEV to tap + (worker id -1)
  because tap devices are by default enumerated from 0, workers from 1
- no check for existing device, let qemu to fail.
- assign different MAC for each worker
  the code looks more cryptic then should, initially I modified only last number of MAC, but that would limit max number of workers with tap networking to 255 and then starts to produce MAC conflicts.