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Make vnd(4) work on sparse files

With the attached diff vnd(4) devices will work on sparse files.
- Make the strategy decision a device flag and set to VNF_USE_VN_RDWR for
  files known to be sparse.
- Change handle_with_rdwr() to use POSIX_FADV_NOREUSE advise to disable
  read ahead.
- Keep the amount of mapped pages below 1 MByte.

Manuel Bouyer was so kind to test a NetBSD install with a PV domU backed
by a sparse file and it worked without problems.

Any objections?

Btw.: Why do we still need handle_with_strategy()?

Juergen Hannken-Illjes - - TU Braunschweig 

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