xml: create an RNG file for common types and move some definitions there

xml: create an RNG file for common types and move some definitions there

domain.rng, network.rng, and interface.rng already use a few of the
same types (or in some cases *should* but don't), and an upcoming code
change will have them sharing even more. To prepare for that, this
patch takes those common data type definitions and moves them into
basictypes.rng.

This may break some rule about the need to RNG files to be autonomous
or something, but I saw that storageencryption.rng is used in this
way, so I figured it must not be completely against the law...