virtio: introduce the consistent-qp feature

virtio: introduce the consistent-qp feature

In certain usecases related to Linux legacy pinning of flows on queue
pairs, it is desirable that, for a given index, the rx and tx virtio
queue be handled by the same worker. This change introduces a flag for
virtio and tap interfaces that allow such a mapping.
Example with two workers rxq 0 and txq 0 on worker 0
         rxq 1 and txq 1 on worker 1
         txq 2 on main thread

Change-Id: I1b74a4788843fd1d0e8dcb4e9da30e609e088fe3
Signed-off-by: Mohammed Hawari <mohammed@hawari.fr>
Type: improvement