virtio-9p: use accessor to get thread_pool

The aio_context_new() function does not allocate a thread pool. This is
deferred to the first call to the aio_get_thread_pool() accessor. It is
hence forbidden to access the thread_pool field directly, as it may be
NULL. The accessor *must* be used always.

Fixes: ebac1202c9
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Tested-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: qemu-stable@nongnu.org
Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
(cherry picked from commit 4b3a4f2d45)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/9pfs/virtio-9p-coth.c

@@ -36,6 +36,6 @@ static int coroutine_enter_func(void *arg)
 void co_run_in_worker_bh(void *opaque)
 {
     Coroutine *co = opaque;
-    thread_pool_submit_aio(qemu_get_aio_context()->thread_pool,
+    thread_pool_submit_aio(aio_get_thread_pool(qemu_get_aio_context()),
                            coroutine_enter_func, co, coroutine_enter_cb, co);
 }