As the buildid is read from /sys/kernel/notes, then if we use perf kvm
buildid-list with a perf data file captured by perf kvm record with
--guestkallsyms and --guestmodules, there is no result in output.

This patch add a explanation about it and add a limit of using perf kvm
buildid-list.
Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/d605a805486340b53bc261aa64d7632ad0a8cf53.1386197481.git.yangds.fnst@cn.fujitsu.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>