cpustats: collect VM user and sys times

cpustats: collect VM user and sys times

As documented in linux.git/Documentation/cgroups/cpuacct.txt,
cpuacct.stat returns user and system time in ticks (the same
unit used in times(2)).  It would be a bit nicer if it were like
getrusage(2) and reported timeval contents, or like cpuacct.usage
and in nanoseconds, but we can't be picky.

* src/util/cgroup.h (virCgroupGetCpuacctStat): New function.
* src/util/cgroup.c (virCgroupGetCpuacctStat): Implement it.
(virCgroupGetValueStr): Allow for multi-line files.
* src/libvirt_private.syms (cgroup.h): Export it.