Fix vectored aio bounce handling immediate errors (Avi Kivity)

Fix vectored aio bounce handling immediate errors (Avi Kivity)

If a bounced vectored aio fails immediately (the inner aio submission
returning NULL) then the bounce handler erronously returns an aio
request which will never be completed (and which crashes when cancelled).

Fix by detecting that the inner request has failed and propagating the
error.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6892 c046a42c-6fe2-441c-8c8c-71466251a162