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

[Richard W.M. Jones]

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.