Add common BusState

Add common BusState

Implement and use a common device bus state.  The main side-effect is
that creating a bus and attaching it to a parent device are no longer
separate operations.  For legacy code we allow a NULL parent, but that
should go away eventually.

Also tweak creation code to veriry theat a device in on the right bus.

Signed-off-by: Paul Brook <paul@codesourcery.com>