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

I'm not sure which version we're up to now. Anyway I believe this addresses all the points that Eric raised in: https://www.redhat.com/archives/libguestfs/2019-March/msg00038.html https://www.redhat.com/archives/libguestfs/2019-March/msg00040.html In particular: - default state of extents_map is all allocated disk - support hole + non-zero - you can now iterate with bounds - removed the nbdkit_extents_clear function; the name is confusing and it doesn't seem like it would be useful - lots of doc changes and clarifications Rich.