[Libguestfs] [PATCH nbdkit 0/2] Generalize the tmpdisk plugin.

Patch 1/2 is uncontroversial. Patch 2/2 is an interesting idea I had to generalize this plugin. It already uses a complete embedded shell script to do most of the work. What if, instead of making special cases for "type" and "label" params, we simply turn any other plugin parameters into script variables? This part of it works fine. However there is another problem which is noted in the "XXX" comment in the second commit message. Rich.