aio-posix: make AioHandler deletion O(1)

aio-posix: make AioHandler deletion O(1)

It is not necessary to scan all AioHandlers for deletion.  Keep a list
of deleted handlers instead of scanning the full list of all handlers.

The AioHandler->deleted field can be dropped.  Let's check if the
handler has been inserted into the deleted list instead.  Add a new
QLIST_IS_INSERTED() API for this check.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Sergio Lopez <[email protected]>
Message-id: 20200214171712[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>