Workaround for backup dialog modeless behaviour

Since now it's modeless, user could have already it opened
but we have to destroy it and recreate when closed for vm list
being refreshed (at least for now)

qubesmanager/qube_manager.py

@@ -336,6 +336,8 @@ class VmManagerWindow(ui_qubemanager.Ui_VmManagerWindow, QtGui.QMainWindow):
         self.sort_by_column = "Type"
         self.sort_order = QtCore.Qt.AscendingOrder
 
+        self.backup_window = None
+
         self.vms_list = []
         self.vms_in_table = {}
 
@@ -1047,9 +1049,17 @@ class VmManagerWindow(ui_qubemanager.Ui_VmManagerWindow, QtGui.QMainWindow):
     # noinspection PyArgumentList
     @QtCore.pyqtSlot(name='on_action_backup_triggered')
     def action_backup_triggered(self):
-        backup_window = backup.BackupVMsWindow(self.qt_app, self.qubes_app,
-                                                self.dispatcher)
-        backup_window.show()
+        if not self.backup_window:
+            self.backup_window = backup.BackupVMsWindow(self.qt_app, self.qubes_app,
+                                                self.dispatcher, self)
+            self.backup_window.finished.connect(self._backup_finished)
+
+        self.backup_window.show()
+        self.backup_window.activateWindow()
+
+    def _backup_finished(self):
+        self.backup_window.deleteLater()
+        self.backup_window = None
 
     # noinspection PyArgumentList
     @QtCore.pyqtSlot(name='on_action_exit_triggered')