linux-aio: simplify removal of completed iocbs from the list

There is no need to do another O(n) pass on the list; the iocb to
split the list at is already available through the array we passed to
io_submit.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1418305950-30924-6-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>