xenpm tool enhancements

xenpm tool enhancements

Two enhancements are made in this patch:

1. Make cpuid in command line optional: if user does not provide
cpuid, xenpm will operate on all the CPUs. this will easy the xenpm
usage. e.g. to set the cpufreq governor, user usually want to set the
governor on all CPUs. User can accomplish this easily with this
enhancement.

2. Add another option "xenpm start" to calculate the Cx/Px state
residency percentage during specified period. when user issue "xenpm
start", xenpm will start to record the Cx/Px state residency. when
user issue CTRL-C, xenpm will stop and show the Cx/Px state residency
and percentage of this period. this is helpful to measure the xenpm
effect for speficied workload.

Signed-off-by: Guanqun Lu <guanqun.lu@intel.com>Signed-off-by: Ke Yu <ke.yu@intel.com>