migration: Abstract the number of bytes sent

Right now we use the "position" inside the QEMUFile, but things like
RDMA already do weird things to be able to maintain that counter
right, and multifd will have some similar problems.
Signed-off-by: NJuan Quintela <quintela@redhat.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

migration/migration.c

@@ -2704,9 +2704,15 @@ static MigThrError migration_detect_error(MigrationState *s)
     }
 }
 
+/* How many bytes have we transferred since the beggining of the migration */
+static uint64_t migration_total_bytes(MigrationState *s)
+{
+    return qemu_ftell(s->to_dst_file);
+}
+
 static void migration_calculate_complete(MigrationState *s)
 {
-    uint64_t bytes = qemu_ftell(s->to_dst_file);
+    uint64_t bytes = migration_total_bytes(s);
     int64_t end_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
     int64_t transfer_time;
 
@@ -2729,13 +2735,15 @@ static void migration_update_counters(MigrationState *s,
                                       int64_t current_time)
 {
     uint64_t transferred, time_spent;
+    uint64_t current_bytes; /* bytes transferred since the beginning */
     double bandwidth;
 
     if (current_time < s->iteration_start_time + BUFFER_DELAY) {
         return;
     }
 
-    transferred = qemu_ftell(s->to_dst_file) - s->iteration_initial_bytes;
+    current_bytes = migration_total_bytes(s);
+    transferred = current_bytes - s->iteration_initial_bytes;
     time_spent = current_time - s->iteration_start_time;
     bandwidth = (double)transferred / time_spent;
     s->threshold_size = bandwidth * s->parameters.downtime_limit;
@@ -2754,7 +2762,7 @@ static void migration_update_counters(MigrationState *s,
     qemu_file_reset_rate_limit(s->to_dst_file);
 
     s->iteration_start_time = current_time;
-    s->iteration_initial_bytes = qemu_ftell(s->to_dst_file);
+    s->iteration_initial_bytes = current_bytes;
 
     trace_migrate_transferred(transferred, time_spent,
                               bandwidth, s->threshold_size);