They're generic concepts, so hoist them.  This also avoids accessor
functions (though kept around for merge with DaveM's net tree).

This goes even further than Jason Wang's 17bb6d40 patch
("virtio-ring: move queue_index to vring_virtqueue") which moved the
queue_index from the specific transport.
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>