vnc: auto assian an id when calling change vnc qmp interface

Only in this way, change vnc qmp interface can take effect,
because qemu_opts_find(&qemu_vnc_opts, id) will return NULL
in vnc_display_open(), It can't connect successfully vnc
server forever.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

qmp.c

@@ -387,6 +387,11 @@ static void qmp_change_vnc_listen(const char *target, Error **errp)
         qemu_opts_del(opts);
     }
     opts = vnc_parse_func(target);
+    if (!opts) {
+        return;
+    }
+
+    vnc_auto_assign_id(olist, opts);
     vnc_display_open("default", errp);
 }