perf evlist: Add a method to return the list of evsels as a string

perf evlist: Add a method to return the list of evsels as a string

Add a 'scnprintf' method to obtain the list of evsels in a evlist as a
string, excluding the "dummy" event used for things like receiving
metadata events (PERF_RECORD_FORK, MMAP, etc) when synthesizing
preexisting threads.

Will be used to improve the error message for workload failure in 'perf
record.

Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/20210414131628.2064862-2-acme@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>