So that internal iothread users can explicitly stop one iothread without
destroying it.

Since at it, fix iothread_stop() to allow it to be called multiple
times.  Before this patch we may call iothread_stop() more than once on
single iothread, while that may not be correct since qemu_thread_join()
is not allowed to run twice.  From manual of pthread_join():

  Joining with a thread that has previously been joined results in
  undefined behavior.

Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-id: 20170928025958.1420-4-peterx@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>