vhost: core dump on quit with worker threads

vhost: core dump on quit with worker threads

Patch 6347 removed the socket file when the interface is deleted and
when VPP process is exitting. The CLI for deleting the interface has
builtin vlib_worker_thread_barrier_sync to prevent the worker
threads from running. Unfortunately, the CLI quit does not have the
builtin vlib_worker_thread_barrier_sync. As a result, it may cause the
worker thread to crash.

The fix is to add the vlib_worker_thread_barrier_sync in vhost_user_exit.

Change-Id: I1eff81170e131098f1799662f0ab48d6fca3def7
Signed-off-by: Steven <sluong@cisco.com>