x86/cpuidle: get accurate C0 value with xenpm tool

x86/cpuidle: get accurate C0 value with xenpm tool

When checking the ACPI funciton of C-status, after 100 seconds sleep,
the sampling value of C0 status from the xenpm tool decreases.
Because C0=NOW()-C1-C2-C3-C4, when NOW() value is during idle time,
NOW() value is bigger than last C-status update time, and C0 value
is also bigger than ture value. if margin of the second error cannot
make up for margin of the first error, the value of C0 would decrease.

Signed-off-by: Huaitong Han <huaitong.han@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>