When updating used ring, the id in used element should be the
index of the first desc in the desc chain.
Fixes: f9b9d1a55775 ("net/virtio-user: add multiple queues in device emulation")
Cc: stable@dpdk.org
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>