Virtual Machine time accounting

The aim of these four patches is to introduce Virtual Machine time accounting.

[PATCH 1/4] as modern CPUs introduce a third running state, after "user" and
"system", we need a new field, "guest", in cpustat to store the time used by
the CPU to run virtual CPU. Modify /proc/stat to display this new field.

[PATCH 2/4] like for cpustat, introduce the "guest" and "cguest" fields for the
tasks. Modify signal_struct and task_struct. Modify /proc/<pid>/stat to display
these new field

[PATCH 3/3] introduce "account modifiers" mechanism in the kernel allowing a
module to modify the collected accounting for a given task. This implementation
is based on the "preempt_notifier". "account_system_time()" and
"account_user_time()" can call functions registered by a module to modify the
cputime value.

[PATCH 4/4] Modify KVM to use the "account modifiers". KVM can now measure time
consumed by a Virtual Machine on a per-cpu basis and modify kernel statistics to
report this value.

Signed-off-by: Laurent Vivier <Laurent.Vivier@bull.net>
-- 
------------- Laurent.Vivier@bull.net  --------------
          "Software is hard" - Donald Knuth

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