[Libguestfs] [PATCH nbdkit] server: Implement extents/can_extents calls for plugins and filters.

[Richard W.M. Jones]

Second version based on nbdkit_extent* calls, as discussed here:

  https://www.redhat.com/archives/libguestfs/2019-March/msg00033.html

Note in particular there is some subtlety about how filters would work
in this implementation.  See docs/nbdkit-filter.pod for the details.

Rich.