migration: use qemu_ftell to compute bandwidth

Prepare for when s->bytes_xfer will be removed.
Reviewed-by: NOrit Wasserman <owasserm@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

migration.c

@@ -589,6 +589,7 @@ static void *migration_thread(void *opaque)
     MigrationState *s = opaque;
     int64_t initial_time = qemu_get_clock_ms(rt_clock);
     int64_t sleep_time = 0;
+    int64_t initial_bytes = 0;
     int64_t max_size = 0;
     int64_t start_time = initial_time;
     bool old_vm_running = false;
@@ -629,7 +630,7 @@ static void *migration_thread(void *opaque)
         }
         current_time = qemu_get_clock_ms(rt_clock);
         if (current_time >= initial_time + BUFFER_DELAY) {
-            uint64_t transferred_bytes = s->bytes_xfer;
+            uint64_t transferred_bytes = qemu_ftell(s->file) - initial_bytes;
             uint64_t time_spent = current_time - initial_time - sleep_time;
             double bandwidth = transferred_bytes / time_spent;
             max_size = bandwidth * migrate_max_downtime() / 1000000;
@@ -646,6 +647,7 @@ static void *migration_thread(void *opaque)
             s->bytes_xfer = 0;
             sleep_time = 0;
             initial_time = current_time;
+            initial_bytes = qemu_ftell(s->file);
         }
         if (qemu_file_rate_limit(s->file)) {
             /* usleep expects microseconds */