vhost: fix desc access in relay helpers
vhost: fix desc access in relay helpers

Descs in desc table should be indexed using the desc idx
instead of the idx of avail ring and used ring.

Fixes: b13ad2decc83 ("vhost: provide helpers for virtio ring relay")

Signed-off-by: Tiwei Bie <f338cf29d7d69ff1bc99f755ebfb0eb0accd409e@intel.com>
Reviewed-by: Maxime Coquelin <9a2667dee3b90866bbc9c2696cff2b7d5616c95a@redhat.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk