display: only un-constrain display size once it is mapped

Wait until the widget is actually on screen before removing its
size constrain. This solves 50x50 window secondary window size
when connecting to a multi-monitor spice guest.

src/virt-viewer-display.c

@@ -448,6 +448,7 @@ end:
      */
     if (priv->dirty) {
         g_idle_add(virt_viewer_display_idle, widget);
+        if (gtk_widget_get_mapped(widget))
             priv->dirty = FALSE;
     }
 }