perf kvm: Clarify the 'perf kvm' -i and -o command line options

The 'perf kvm' subcommand has options that it in turn passes to other
perf subcommands such as 'report' and 'record', particularly -i and -o
end up setting the same variable that will then be used for 'record's -o
and report '-i', which ends up being confusing, leading some to think
that both -i and -o can be used with 'report'.

Improve the man page to state that -i is used with the post-processing
subcommands while -o is used just with 'record' and that to save the
output of 'report' one should simply redirect its output to a file.

Noticed while reading the https://www.linux-kvm.org/page/Perf_events
page.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Steve Dickson <steved@redhat.com>
Cc: William Cohen <wcohen@redhat.com>
Link: https://lkml.kernel.org/n/tip-tclbttvmgtm525fvmh85f7d9@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>