The Linux AIO userspace ABI includes a ring that is shared with the
kernel.  This allows userspace programs to process completions without
system calls.

Add an AioContext poll handler to check for completions in the ring.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20161201192652.9509-6-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>