Manager: Warn on close when machines are running

src/qt/qt_vmmanager_model.hpp

@@ -57,6 +57,7 @@ public:
     void reload(QWidget* parent = nullptr);
     void updateDisplayName(const QModelIndex &index, const QString &newDisplayName);
     QHash <QString, int> getProcessStats();
+    int getActiveMachineCount();
 signals:
     void systemDataChanged();