vl: Don't continue after -smp help.

We continue after -smp help:

    $ qemu-system-x86_64 -smp help -display none -monitor stdio
    smp-opts options:
      cores=<num>
      cpus=<num>
      dies=<num>
      maxcpus=<num>
      sockets=<num>
      threads=<num>
    QEMU 6.0.50 monitor - type 'help' for more information
    (qemu)

Other options, such as -object help and -device help, don't.

Adjust -smp not to continue either.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-17-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>

softmmu/vl.c

@@ -1545,7 +1545,7 @@ machine_parse_property_opt(QemuOptsList *opts_list, const char *propname,
     prop = keyval_parse(arg, opts_list->implied_opt_name, &help, errp);
     if (help) {
         qemu_opts_print_help(opts_list, true);
-        return;
+        exit(0);
     }
     opts = qdict_new();
     qdict_put(opts, propname, prop);