aio_poll(ctx, true) will soon block if any fd handlers have been set.
Previously it would only block when .io_flush() returned true.

This means that callers must check their wait condition *before*
aio_poll() to avoid deadlock.
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>