vhost: fix dead lock on closing in server mode

vhost: fix dead lock on closing in server mode

When qemu close the unix socket fd of the vhostuser as a
server, and then immediately delete the vhostuser port on
openvswitch. There will be a deadlock.

A thread (fdset event thread):       B thread:
1. fdset_event_dispatch              rte_vhost_driver_unregister
2. set the fd busy to 1.             lock vsocket->conn_mutex
3. vhost_user_read_cb                fdset_del waits busy changed to 0.
4. vhost peer closed, remove the
   conn from vsocket->conn_list:
   lock vsocket->conn_mutex

5. set the fd busy to 0

Fixes: 65388b43f592 ("vhost: fix fd leaks for vhost-user server mode")
Cc: stable@dpdk.org
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Acked-by: Maxime Coquelin <maxime.coquelin@redhat.com>