[bus,driver] Move attach_fdo into driver module

[bus,driver] Move attach_fdo into driver module

attach_fdo is really a _driver_ thing, rather than a
_bus_ thing, so we move it into the driver module as
driver__attach_fdo_().

This allows us to remove bus module initialization,
meaning the bus module should be purely library now.

Our driver's AddDevice routine (driver__attach_fdo_) is
hard-coded to only attach a maximum of one FDO, which
will be the main WinVBlock bus.  This could change later.