kvm tools: Prepare support for VIRTIO_RING_F_EVENT_IDX

kvm tools: Prepare support for VIRTIO_RING_F_EVENT_IDX

This patch is the base for enabling support for event index feature in the virtio spec.
We do so by updating and evaluating the used/avail event idx in the virtio ring functions.

Actual usage of this flag is in the following patches.

The results are less notifications between the guest and host, and in
result faster operation of the virt queues.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>