Virtio-net: raise interrupt for TX queue used descriptors

[sandreim]

This is a regression dating back to this PR: #413.

If the TX queue becomes full the guest would not be able to enqueue any TX packets until an RX from the tap device will raise the interrupt and the guest will inspect both RX/TX queue status.
The issue impacts the workloads where the TX:RX ratio is highly skewed towards TX, for example sending large bursts of traffic without doing much RX.

Before merging a fix benchmarking is needed to understand the performance implications of raising additional irqs.

TODO:

• Implement irq raising for used descriptors (TX)
• Performance benchmark
• Integration test

[sandreim]

Fixed in PR #1444