Fix race in POSIX AIO emulation (Jan Kiszka)

Fix race in POSIX AIO emulation (Jan Kiszka)

When we cancel an AIO request that is already being processed by
aio_thread, qemu_paio_cancel should return QEMU_PAIO_NOTCANCELED as long
as aio_thread isn't done with this request. But as the latter currently
updates aiocb->ret after every block of the request, we may report
QEMU_PAIO_ALLDONE too early.

Futhermore, in case some zero-length request should have been queued,
aiocb->ret is never set to != -EINPROGRESS and callers like
raw_aio_cancel could get stuck in an endless loop.

Fix those issues by updating aiocb->ret _after_ the request has been
fully processed. This also simplifies the locking.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6278 c046a42c-6fe2-441c-8c8c-71466251a162