Added additional check for cloning a VM

An attempt to clone a VM to a name that is already in use will
fail more gracefully and with an explicit warning message.

qubesmanager/qube_manager.py

@@ -852,6 +852,15 @@ class VmManagerWindow(ui_qubemanager.Ui_VmManagerWindow, QtGui.QMainWindow):
         if not ok or clone_name == "":
             return
 
+        name_in_use = clone_name in self.qubes_app.domains
+
+        if name_in_use:
+            QtGui.QMessageBox.warning(
+                None, self.tr("Name already in use!"),
+                self.tr("There already exists a qube called '{}'. "
+                        "Cloning aborted.").format(clone_name))
+            return
+
         self.progress = QtGui.QProgressDialog(
             self.tr(
                 "Cloning Qube..."), "", 0, 0)