perf kvm: Introduce option -v for perf kvm command.

As there is no -v option for perf kvm, the all debug message for perf
kvm will nerver be printed out to user.

Example:
	# perf kvm --guestmount /tmp/guestmount/ record -a
	Not enough memory for reading perf file header

It is confusing message for newbies such as me. With this patch applied,
we can use -v option to get the detail.

Example:
	# perf kvm --guestmount /tmp/guestmount/ record -a -v
	Can't access file /tmp/guestmount//15069/proc/kallsyms
	Not enough memory for reading perf file header
Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Cc: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/r/1386609311-23889-1-git-send-email-yangds.fnst@cn.fujitsu.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-kvm.txt

@@ -10,9 +10,9 @@ SYNOPSIS
 [verse]
 'perf kvm' [--host] [--guest] [--guestmount=<path>
 	[--guestkallsyms=<path> --guestmodules=<path> | --guestvmlinux=<path>]]
-	{top|record|report|diff|buildid-list}
+	{top|record|report|diff|buildid-list} [<options>]
 'perf kvm' [--host] [--guest] [--guestkallsyms=<path> --guestmodules=<path>
-	| --guestvmlinux=<path>] {top|record|report|diff|buildid-list|stat}
+	| --guestvmlinux=<path>] {top|record|report|diff|buildid-list|stat} [<options>]
 'perf kvm stat [record|report|live] [<options>]
 
 DESCRIPTION
@@ -93,6 +93,9 @@ OPTIONS
 	kernel module information. Users copy it out from guest os.
 --guestvmlinux=<path>::
 	Guest os kernel vmlinux.
+-v::
+--verbose::
+	Be more verbose (show counter open errors, etc).
 
 STAT REPORT OPTIONS
 -------------------

tools/perf/builtin-kvm.c

@@ -1690,6 +1690,8 @@ int cmd_kvm(int argc, const char **argv, const char *prefix __maybe_unused)
 			   "file", "file saving guest os /proc/kallsyms"),
 		OPT_STRING(0, "guestmodules", &symbol_conf.default_guest_modules,
 			   "file", "file saving guest os /proc/modules"),
+		OPT_INCR('v', "verbose", &verbose,
+			    "be more verbose (show counter open errors, etc)"),
 		OPT_END()
 	};