perf tools: Use machine->pid for tgid if machine is guest.

When we synthesize an comm event, if machine is guest, we should
use the pid of machine as the event->comm.pid, rather than tgid
of thread.

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/22455abe107c618a361e7b667ad0f098f7c9b4a3.1387572416.git.yangds.fnst@cn.fujitsu.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>