perf tools: Find the proc info under machine->root_dir.

When we synthesize the threads, we are looking for the infomation under
/proc. But it is only for host.

This patch look for the path of proc under machine->root_dir, then
XXX__synthesize_threads() functions can support guest machines.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Link: http://lkml.kernel.org/r/927b937da9177a079abafe4532fa9c9b60b5c4b7.1387572416.git.yangds.fnst@cn.fujitsu.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>