Set fallback mode tip message

The registry table string type does not support line return. If a line return is required, "backslash n" should be used to add the line return. The length of the content string cannot be greater than 1024 characters. Displays a user-defined message when the VDI plugin is not usable and fallback mode is set to 1, 2, 5, 6, or 7. When this registry key does not exist or the string value is empty, no dialog message will be shown.