percpu_xmit: cleanup

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
percpu_xmit::xmit(): fix an "if" on xmit_prep() exit code

NetDevTxq::xmit_prep() should return 0 zero on success and error codes
may be different - EINVAL is one of them. Thus the right thing to do is
to check the exit code to be non-zero.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
virtio::net::txq: Added some debug statistics.

Uncomment #define DEBUG_VIRTIO_TX and rebuild and then running ifconfig will
show some interesting Tx related statistics:
    - HV kicks caused by Tx.
    - Number of packets handled from the worker.
    - Number of worker wakeups.
    - Some interesting ratios.

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