lguest: set max_pfn_mapped, growl loudly at Yinghai Lu

6af61a76 'x86: clean up max_pfn_mapped
usage - 32-bit' makes the following comment:

    XEN PV and lguest may need to assign max_pfn_mapped too.

But no CC.  Yinghai, wasting fellow developers' time is a VERY bad
habit.  If you do it again, I will hunt you down and try to extract
the three hours of my life I just lost :)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Yinghai Lu <yhlu.kernel@gmail.com>