virsh: omit OPTION section in 'virsh help' if no option exists

Don't print 'OPTION' if there's no options. Just behaves as DESCRIPTION
does.
This mostly affects 'interface' command group.
Signed-off-by: NZhang Xiaohe <zhangxh@cn.fujitsu.com>
Reported-by: NLi Yang <liyang.fnst@cn.fujitsu.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

tools/virsh.c

@@ -1268,7 +1268,7 @@ vshCmddefHelp(vshControl *ctl, const char *cmdname)
             fprintf(stdout, "    %s\n", _(desc));
         }
 
-        if (def->opts) {
+        if (def->opts && def->opts->name) {
             const vshCmdOptDef *opt;
             fputs(_("\n  OPTIONS\n"), stdout);
             for (opt = def->opts; opt->name; opt++) {