hw/display/cirrus_vga: Update the documentation URL

The documentation URL is not working, but is backed up by the
Wayback Machine on the Internet Archive.
Replace the outdated link by a captured one.
Add another link to the VGADOC4b.ZIP archive content.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190504121650.12651-1-philmd@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
1 file changed