migration: cleanup stats update into function

We have quite a few lines in migration_thread() that calculates some
statistics for the migration interations.  Isolate it into a single
function to improve readability.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/migration.h

@@ -102,6 +102,17 @@ struct MigrationState
     QEMUBH *cleanup_bh;
     QEMUFile *to_dst_file;
 
+    /* bytes already send at the beggining of current interation */
+    uint64_t iteration_initial_bytes;
+    /* time at the start of current iteration */
+    int64_t iteration_start_time;
+    /*
+     * The final stage happens when the remaining data is smaller than
+     * this threshold; it's calculated from the requested downtime and
+     * measured bandwidth
+     */
+    int64_t threshold_size;
+
     /* params from 'migrate-set-parameters' */
     MigrationParameters parameters;