mirror: switch mirror_iteration to AIO

There is really no change in the behavior of the job here, since
there is still a maximum of one in-flight I/O operation between
the source and the target.  However, this patch already introduces
the AIO callbacks (which are unmodified in the next patch)
and some of the logic to count in-flight operations and only
complete the job when there is none.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>