Add some debugging support to clientside TBScriptLock; use it in libvnode_xen.
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.
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