qemu-storage-daemon: Add vhost-user-blk help

Add missing vhost-user-blk help:

  $ qemu-storage-daemon -h
  ...
    --export [type=]vhost-user-blk,id=<id>,node-name=<node-name>,
             addr.type=unix,addr.path=<socket-path>[,writable=on|off]
             [,logical-block-size=<block-size>][,num-queues=<num-queues>]
                           export the specified block node as a
                           vhosts-user-blk device over UNIX domain socket
    --export [type=]vhost-user-blk,id=<id>,node-name=<node-name>,
             fd,addr.str=<fd>[,writable=on|off]
             [,logical-block-size=<block-size>][,num-queues=<num-queues>]
                           export the specified block node as a
                           vhosts-user-blk device over file descriptor
  ...

Fixes: 90fc91d50b7 ("convert vhost-user-blk server to block export API")
Reported-by: Qing Wang <qinwang@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220107105420.395011-3-f4bug@amsat.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

storage-daemon/qemu-storage-daemon.c

@@ -104,6 +104,19 @@ static void help(void)
 "                         export the specified block node over FUSE\n"
 "\n"
 #endif /* CONFIG_FUSE */
+#ifdef CONFIG_VHOST_USER_BLK_SERVER
+"  --export [type=]vhost-user-blk,id=<id>,node-name=<node-name>,\n"
+"           addr.type=unix,addr.path=<socket-path>[,writable=on|off]\n"
+"           [,logical-block-size=<block-size>][,num-queues=<num-queues>]\n"
+"                         export the specified block node as a\n"
+"                         vhost-user-blk device over UNIX domain socket\n"
+"  --export [type=]vhost-user-blk,id=<id>,node-name=<node-name>,\n"
+"           fd,addr.str=<fd>[,writable=on|off]\n"
+"           [,logical-block-size=<block-size>][,num-queues=<num-queues>]\n"
+"                         export the specified block node as a\n"
+"                         vhost-user-blk device over file descriptor\n"
+"\n"
+#endif /* CONFIG_VHOST_USER_BLK_SERVER */
 "  --monitor [chardev=]name[,mode=control][,pretty[=on|off]]\n"
 "                         configure a QMP monitor\n"
 "\n"