mirror: fix early wake from sleep due to aio

The mirror blockjob coroutine rate-limits itself by sleeping.  The
coroutine also performs I/O asynchronously so it's important that the
aio callback doesn't wake the coroutine early as that breaks
rate-limiting.

Reported-by: Joaquim Barrera <jbarrera@ac.upc.edu>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
