hvm: Fix 2 type mismatches in vlapic.h and hpet.c for 32-bit build Xen

hvm: Fix 2 type mismatches in vlapic.h and hpet.c for 32-bit build Xen

For 32-bit build of Xen:
1) the first mismatch (in hpet_read(), length is 4)  makes guest think
the HPET DM is buggy (we return 0 for HPET_CFG.COUNTER_CLK_PERIOD to
guest), so guest wouldn't use HPET at all.
2) the second one: if tmict is 62500000 and timer_divisor is 16
(Fedoar7's installer uses the values at some time), 10 * 62500000 * 16
= 0x2540BE400 -- it's too big to be held in uint32_t.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>=20