sched, cpuacct: Fix niced guest time accounting

CPU time of a guest is always accounted in 'user' time
without concern for the nice value of its counterpart
process although the guest is scheduled under the nice
value.

This patch fixes the defect and accounts cpu time of
a niced guest in 'nice' time as same as a niced process.

And also the patch adds 'guest_nice' to cpuacct. The
value provides niced guest cpu time which is like 'nice'
to 'user'.

The original discussions can be found here:

  http://www.mail-archive.com/kvm@vger.kernel.org/msg23982.html
  http://www.mail-archive.com/kvm@vger.kernel.org/msg23860.html

Signed-off-by: Ryota Ozaki <ozaki.ryota@gmail.com>
Acked-by: Avi Kivity <avi@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1256314810-7897-1-git-send-email-ozaki.ryota@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/kernel_stat.h

@@ -25,6 +25,7 @@ struct cpu_usage_stat {
 	cputime64_t iowait;
 	cputime64_t steal;
 	cputime64_t guest;
+	cputime64_t guest_nice;
 };
 
 struct kernel_stat {