fix bdrv_aio_read API breakage in qcow2 (Andrea Arcangeli)

fix bdrv_aio_read API breakage in qcow2 (Andrea Arcangeli)

I noticed the qemu_aio_flush was doing nothing at all. And a flood of
cmd_writeb commands leading to a noop-invocation of qemu_aio_flush
were executed.

In short all 'memset;goto redo' places must be fixed to use the bh and
not to call the callback in the context of bdrv_aio_read or the
bdrv_aio_read model falls apart. Reading from qcow2 holes is possible
with phyisical readahead (kind of breada in linux buffer cache).

This is needed at least for scsi, ide is lucky (or it has been
band-aided against this API breakage by fixing the symptom and not the
real bug).

Same bug exists in qcow of course, can be fixed later as it's less
urgent.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5574 c046a42c-6fe2-441c-8c8c-71466251a162