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

tools/perf/builtin-diff.c

@@ -1000,8 +1000,7 @@ static int data_init(int argc, const char **argv)
 			data__files_cnt = argc;
 			use_default = false;
 		}
-	} else if (symbol_conf.default_guest_vmlinux_name ||
-		   symbol_conf.default_guest_kallsyms) {
+	} else if (perf_guest) {
 		defaults[0] = "perf.data.host";
 		defaults[1] = "perf.data.guest";
 	}