iotests: Add regresion test case for write notifier assertion failure

The idea is to let the top level bs have a big request alignment with
blkdebug, so that the aio_write request issued from monitor will be
serialised. This tests that QEMU doesn't crash upon the read request
from the backup job's write notifier, which is a very special case of
"reentrant" request.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1448962590-2842-4-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>