perf kvm: Make perf kvm diff support --guestmount.

In manpage of perf-kvm, --guestmount is supported by diff command, but
it does not work well.

This patch change the extend the checking in buildid-diff from
guestkallsyms or guestmodules to perf_guest. Then this checking can
cover the all cases perf kvm is used for.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Link: http://lkml.kernel.org/r/72857ed89642e0633f5e88f7e7abbc9645359e8e.1386368672.git.yangds.fnst@cn.fujitsu.com


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