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

[Richard W.M. Jones]

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.