Double count Guest Time in Total CPU Time

[jimmyxian]

When running Virtual Machine(KVM) in host, I find that the value of host cpu usage is different between falcon agent and top cmd. Actually, falcon agent use nux package to calculate cpu usage.
https://github.com/open-falcon-archive/agent/blob/master/funcs/cpustat.go#L19

In Linux Kernel, User time is already include Guest Time. So when read cpu info from /proc/stat, we should not add guest time once more.

https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/sched/cputime.c#n138