perf kvm: Fix subcommand matching error

Currently the 'diff', 'top', 'buildid-list' and 'stat' perf commands use
strncmp() to match subcommands. As a result, matching does not meet
expectation.

For example:
# perf kvm diff1234
# Event 'cycles'
#
# Baseline Delta Abs Shared Object Symbol
# ........ ......... ............. ......
#

# Event 'dummy:HG'
#
# Baseline Delta Abs Shared Object Symbol
# ........ ......... ............. ......
#
# echo $?
0
#

Invalid information should be returned, but success is actually returned.

Solution: Use strstarts() to match subcommands.

After:
# perf kvm diff1234

Usage: perf kvm [<options>] {top|record|report|diff|buildid-list|stat}

-i, --input <file> Input file name
-o, --output <file> Output file name
-v, --verbose be more verbose (show counter open errors, etc)
--guest Collect guest os data
--guest-code Guest code can be found in hypervisor process
--guestkallsyms <file>
file saving guest os /proc/kallsyms
--guestmodules <file>
file saving guest os /proc/modules
--guestmount <directory>
guest mount directory under which every guest os instance has a subdir
--guestvmlinux <file>
file saving guest os vmlinux
--host Collect host os data

# echo $?
129
#

Signed-off-by: Yang Jihong <[e-mail]>
Acked-by: Namhyung Kim <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-kvm.c

···
 		return __cmd_record(file_name, argc, argv);
 	else if (strlen(argv[0]) > 2 && strstarts("report", argv[0]))
 		return __cmd_report(file_name, argc, argv);
-	else if (!strncmp(argv[0], "diff", 4))
+	else if (strlen(argv[0]) > 2 && strstarts("diff", argv[0]))
 		return cmd_diff(argc, argv);
-	else if (!strncmp(argv[0], "top", 3))
+	else if (!strcmp(argv[0], "top"))
 		return cmd_top(argc, argv);
-	else if (!strncmp(argv[0], "buildid-list", 12))
+	else if (strlen(argv[0]) > 2 && strstarts("buildid-list", argv[0]))
 		return __cmd_buildid_list(file_name, argc, argv);
 #ifdef HAVE_KVM_STAT_SUPPORT
-	else if (!strncmp(argv[0], "stat", 4))
+	else if (strlen(argv[0]) > 2 && strstarts("stat", argv[0]))
 		return kvm_cmd_stat(file_name, argc, argv);
 #endif
 	else