Store and allow libvirt instance device buses and models to be updated

QEMU support for device buses and models can come and go dependent on the
underlying instance machine type *and* QEMU version used within an environment.
The defaults provided by libosinfo and currently hardcoded in to the libvirt
driver are also not persisted by each instance at present.

This spec aims to outline a basic set of nova-manage commands to allow
operators to move instances between specific device bus and model types without
requiring a rebuild.