coroutine: simplify co_aio_sleep_ns() prototype

The AioContext pointer argument to co_aio_sleep_ns() is only used for
the sleep timer.  It does not affect where the caller coroutine is
resumed.

Due to changes to coroutine and AIO APIs it is now possible to drop the
AioContext pointer argument.  This is safe to do since no caller has
specific requirements for which AioContext the timer must run in.

This patch drops the AioContext pointer argument and renames the
function to simplify the API.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20171109102652.6360-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>