perf trace: Extract evsel contructor from perf_evlist__add_pgfault

Prep work for next patches, where we'll need access to the created
evsels, to possibly configure callchains.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-2pcgsgnkgellhlcao4aub8tu@git.kernel.org


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