Otherwise the AioContext will never be released.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 1447419624-21918-1-git-send-email-berto@igalia.com
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>