Make it clear how chained indirect descriptors must work.

Chained indirect descriptors were partially banned by virtio-15, which said how devices would handle them (by terminating the descriptor when it hits the end of the indirect descriptors). It didn't spell out that (obviously) they shouldn't be generated by drivers!

It also leaves open the possibility of one or more direct descriptors followed by a trailing indirect descriptor. All implementations I know of handle this, though none I know of generate such things. My test lguest implementation didn't (now fixed); we should spell out explicitly that it's possible.