lguest: Sanitize the lguest clock.

Now the TSC code handles a zero return from calculate_cpu_khz(),
lguest can simply pass through the value it gets from the Host: if
non-zero, all the normal TSC code applies.

Otherwise (or if the Host really doesn't support TSC), the clocksource
code will fall back to the slower but reasonable lguest clock.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>