tools lib traceevent: Fix reporting of unknown SVM exit reasons

On AMD, exist code -1 is also a possible value, but we use it for
terminating the list of known exit reasons. This leads to EXIT_ERR being
reported for unkown ones. Fix this by using an NULL string pointer as
terminal.

Link: http://lkml.kernel.org/r/[e-mail]
Link: http://lore.kernel.org/linux-trace-devel/[e-mail]

Signed-off-by: Jan Kiszka <[e-mail]>
[ Ported from trace-cmd.git ]
Cc: Andrew Morton <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: [e-mail]
Link: http://lore.kernel.org/lkml/[e-mail]
Signed-off-by: Steven Rostedt (VMware) <[e-mail]>
Signed-off-by: Tzvetomir Stoyanov (VMware) <[e-mail]>
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/lib/traceevent/plugins/plugin_kvm.c

···
 		}
 	if (!strings)
 		return "UNKNOWN-ISA";
-	for (i = 0; strings[i].val >= 0; i++)
+	for (i = 0; strings[i].str; i++)
 		if (strings[i].val == val)
 			break;