virtio::net: Don't print floating point! :D

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
virtio::net: Forgot to cast... ;)

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
virtio::net: prio auto-tuning: make it even more aggressive

- use level8 for a low water mark (instead of level64)
- use level64 for a high watermark (instead of level256)

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
virtio::net: increase the priority of tx_worker when it never goes to sleep

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