aio: convert aio_poll() to g_poll(3)

aio: convert aio_poll() to g_poll(3)

AioHandler already has a GPollFD so we can directly use its
events/revents.

Add the int pollfds_idx field to AioContext so we can map g_poll(3)
results back to AioHandlers.

Reuse aio_dispatch() to invoke handlers after g_poll(3).

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Laszlo Ersek <[email protected]>
Message-id: 1361356113[email protected]
Signed-off-by: Anthony Liguori <[email protected]>