migration: use s->threshold_size inside migration_update_counters

migration: use s->threshold_size inside migration_update_counters

Fixes: b15df1ae50 ("migration: cleanup stats update into function")
The threshold size is changed to be recorded in s->threshold_size.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>