perf machine: Introduce machine__findnew_dso() method

Similar to machine__findnew_thread(), also prepping for refcounting and
locking, this time for struct dso instances.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-fv3tshv5o1413coh147lszjc@git.kernel.org


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