[Libguestfs] [PATCH nbdkit v2] PROPOSED: server: Implement list_exports.

Updated proposal, taking into account the default export. Instead of adding a second call, I made a couple of changes to list_exports: (1) If the plugin has a concept of a default export, it should add it as the first element in the exports list. (2) There is a new default_only flag which tells the plugin that the client is trying to request the name of the default export, so the plugin may return only a single element. However it's fine for plugins to ignore this flag. What about plugins that don't have any concept of a default export? This makes the default export be whatever happens to be first in the list. Not sure if this is a bad thing or not. Rich.