perf machine: Add kernel_dso() method

To reduce boilerplate in some places.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-9s1bgoxxhlnu037e1nqx0tw3@git.kernel.org


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