linux-aio: queue requests that cannot be submitted

linux-aio: queue requests that cannot be submitted

Keep a queue of requests that were not submitted; pass them to
the kernel when a completion is reported, unless the queue is
plugged.

The array of iocbs is rebuilt every time from scratch.  This
avoids keeping the iocbs array and list synchronized.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1418305950-30924-2-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>