[Libguestfs] [PATCH nbdkit 0/8] Implement extents using a simpler array.

[Richard W.M. Jones]

Not sure what version we're up to, but this reimplements extents using
the new simpler structure described in this thread:

  https://www.redhat.com/archives/libguestfs/2019-March/msg00077.html

I also fixed most of the things that Eric pointed out in the previous
review, although I need to go back over his replies and check I've got
everything.

This needs a bit more testing.  However the tests do work.

Rich.