perf tools: Do not synthesize the treads of default guest.

As the default guest is designed to handle orphan kernel symboles with
--guestkallsysms and --guestmodules, it has no user space.

So we should skip synthesizing threads if machine is default guest.

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/e9ddb5dac6f963169657218b12ceb3c2030f54e8.1387572416.git.yangds.fnst@cn.fujitsu.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>