Don't add virtual items to the display

By definition virtual vias should only exist in the backend, not
affecting users

Fixes https://gitlab.com/kicad/code/kicad/issues/8788

pcbnew/router/pns_kicad_iface.cpp

@@ -1406,6 +1406,9 @@ void PNS_KICAD_IFACE_BASE::SetDebugDecorator( PNS::DEBUG_DECORATOR *aDec )
 
 void PNS_KICAD_IFACE::DisplayItem( const PNS::ITEM* aItem, int aClearance, bool aEdit )
 {
+    if( aItem->IsVirtual() )
+        return;
+
     ROUTER_PREVIEW_ITEM* pitem = new ROUTER_PREVIEW_ITEM( aItem, m_view );
 
     if( aClearance >= 0 )