xl: fix 'xl help' command

xl help 'command' should give the help of the 'command',
but current it does not, it just gives a full list of xl
supported commands. This patch fix it.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>

tools/libxl/xl.c

@@ -76,7 +76,7 @@ int main(int argc, char **argv)
 
     if (i >= cmdtable_len) {
         if (!strcmp(cmd, "help")) {
-            help(argv[1]);
+            help(argv[2]);
             exit(0);
         } else {
             fprintf(stderr, "command not implemented\n");