[Libguestfs] [PATCH nbdkit] New tmpdisk plugin.

Unfinished (needs tests). This is my attempt to make a "remote tmpfs" plugin as outlined in this prior email: https://www.redhat.com/archives/libguestfs/2020-March/msg00134.html Although it would be possible to construct something a bit like this using existing plugins and filters (perhaps with some new features in those filters) I think it may be nicer to have a dedicated plugin for this, as it makes the documentation simpler to understand and allows us to implement unique features. In particular notice that this plugin only deals with temporary disks so deliberately ignores flush/FUA, which should make it nice and fast. Rich.