net: added ifi_oqueue_is_full statistics counter

Number of times the packet could not be sent due to a lack of
free space on a HW ring.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
virtio::net: added a tx_hw_queue_is_full statistics

This counter will contain the number of timers the packet was unable to
be sent due to no free space on a HW ring.
This counter is incremented at most once per-packet.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
network.json API: Added a ifi_oqueue_is_full statistics counter to /network/ifconfig/{if} output

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