[Libguestfs] [PATCH NOT WORKING nbdkit] vddk: Restructure plugin to allow greater parallelism.

We had a query yesterday about the VDDK plugin and making it actually obey the weird "Multithreading Considerations" rules in the VDDK documentation (https://vdc-download.vmware.com/vmwb-repository/dcr-public/8f96698a-0e7b-...) This patch is my attempt to implement this. The idea is that the plugin starts a background thread and routes all calls to certain VDDK functions through this thread, ensuring that the "Multithreading Considerations" are obeyed. It doesn't work. Because we start the background thread when the plugin is loaded, when nbdkit forks the thread disappears. It's possible that we need in nbdkit a supported way for plugins to create background threads. In any case the actual benefit of this patch is dubious. Closer reading of the documentation indicates that VDDK does not allow full parallel requests on single handles. The way we normally use VDDK (eg. from virt-v2v) is that we only have a single connection open (and therefore a single handle). So whether we'd gain any benefit in real world cases seems unlikely. We cannot enable multi-conn on writable VDDK handles because the documentation gives us no indication that VDDK provides the required guarantees, although we might consider enabling multi-conn for readonly handles. Rich.