vhost: fix crash on closing in client mode

vhost: fix crash on closing in client mode

when rte_vhost_driver_unregister detstroy the vsocket, we
should set it to NULL after freeing it, because in client mode,
the conn may be added to reconnect thread while vsocket is
destroyed. In one case, if qemu create vhostuser port as a
server with the same unix path, the reconnect thread will
reconnect to it while vsocket is destroyed.

To fix this:
1. set vsocket to NULL after free it.
2. remove the reconnection from reconnection thread in suitable
   position.

Cc: stable@dpdk.org
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>