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