perf machine: Introduce find_thread method

There are cases where we want just to find a thread if it exists
already, so provide a method for that.

While doing that start moving 'machine' methods to a separate file.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-8wpzqs9kfupng6xq8hx6lnxa@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>