virtio::net: Separated logic for prio auto-tuning for Rx and Tx tasks

- Set Rx low/high watermarks to 64/256.
- Set Tx low/high watermarks to 8/64.
- Made the logic in poll_until() to the same method as other prio auto-tunic
  code by extending the semantics of single "wakeup" to handling at most ring
  size of packets.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>