[this one is required for [PATCH] fully split aio_pool from BlockDriver,
 sorry for not sending it out earlier]

Add a qcow_aio_setup helper to qcow to shared common code between
the aio_readv and aio_writev methods.  Based on the function with
the same name in qcow2.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>