hvm: Lower HPET frequency to 1/32 of the TSC.

hvm: Lower HPET frequency to 1/32 of the TSC.

The frequency of HPET device model is defined to be the same as TSC's,
but this doesn't work well with calibrate_tsc_hpet() in Linux kernel
2.6.16-33, causing some IA32 Linux HVM guests to failt o boot.

Calibrate_tsc_hpet() tries to figure out how many HPET ticks a TSC
cycle equals; it magnifies the result by scale of 2^32, trying to get
a more accurate result since it assumes the frequency of HPET in real
world is usually less than 1/100 of TSC, so the result of "(2^32 *
hpet_freq) / tsc_freq" may exceed 32bits, then a "divide error
(overflow)" would occur!

The result doesn't overflow every time because hpet_freq/tsc_freq may
less than 1.0 due to the little inaccuracy in the implementation of
HVM timer virtualization.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>