virtio: Add queue interface to restore avail index from vring used index

In case of backend crash, it is not possible to restore internal
avail index from the backend value as vhost_get_vring_base
callback fails.

This patch provides a new interface to restore internal avail index
from the vring used index, as done by some vhost-user backend on
reconnection.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
