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

This tentative commit implements extents/can_extents, roughly as discussed in the previous thread here: https://www.redhat.com/archives/libguestfs/2019-March/msg00017.html I can't say that I'm a big fan of having the plugin allocate an extents array. There are no other plugin callbacks currently where we require the plugin to allocate complex data structures (or indeed do any allocation at all). The interface is complex and error prone for plugin writers. However I can't at the moment think of a better way to do it. Rich.