Change the various AGP drivers that attach to the Host-PCI bridge device to

Change the various AGP drivers that attach to the Host-PCI bridge device to
attach to the hostb driver instead.  This means that agp can now be loaded
at runtime (in theory at least).  Also, the drivers no longer have to
explicity call device_verbose() to cancel out any earlier calls to
device_quiet() by the hostb(4) driver (this shows a limitation in new-bus,
drivers really shouldn't be doing device_quiet() until they know they are
going to drive that device, i.e. in attach).