aio: Introduce aio_external_disabled

This allows AioContext users to check the enable/disable state of
external clients.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1446177989-6702-2-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
