Bus names are chosen by the system as follows:
* If the driver of the parent device model provides a name, use that.
* Else, if the parent device has id ID, use ID.NUM, where NUM is the bus
number, counting from zero in creation order.
* Else, use TYPE.NUM, where TYPE is derived from the bus type, and NUM
is the bus number, as above.
### Paul proposes to drop ID.NUM.
### Paul proposes to either drop TYPE.NUM (and require drivers to
provide bus names), or make NUM count separately for each bus type.