If the TBScriptLock caller provides a debug message, it will be stored
in a file, and other blocked TBScriptLock callers will get (possibly
slightly racy) info about who holds the lock.

Then, use this in libvnode_xen to get some info about long calls to xl
(create|halt|reboot|etc).

Also enable lockdebug in libvnode_xen for now.