vhost: fix ring index returned to master on stop

vhost: fix ring index returned to master on stop

According to the "Vhost-user Protocol" document,
VHOST_USER_GET_VRING_BASE should get the available vring base offset.

Fixes: 8f972312b8f4 ("vhost: support vhost-user")
Cc: stable@dpdk.org
Signed-off-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Signed-off-by: Tomasz Kulasek <tomaszx.kulasek@intel.com>
Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>