vt: document vc_data by example

All those members of vc_data are each explained in short. But it needs
an example for one to understand the whole picture.

So add an ascii art depicting the most important vc_data members.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>