1. kpartx does not do its own locking, so must protect it from
   concurrent runs during setup.

2. Allow mulitple XEN hosts (up to 4) to set up concurrently via new
   TBScriptLock() shared lock, but revert to exclusive lock if an
   image needs to created or modified.

3. Convert root disk (XEN-STD) to a plain NDZ file from raw LVM bits,
   so that we get some benefit from the buffer cache.