virtio-9p: use accessor to get thread_pool

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: ebac1202c95a4f1b76b6ef3f0f63926fa76e753e
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 4b3a4f2d458ca5a7c6c16ac36a8d9ac22cc253d6)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>