Many PCI host bridges consist of a sysbus device and a PCI device.
You need both for the thing to work.  Arguably, these bridges should
be modelled as a single, composite devices instead of pairs of
seemingly independent devices you can only use together, but we're not
there, yet.

Since the sysbus part can't be instantiated with device_add, yet,
permitting it with the PCI part is useless.  We shouldn't offer
useless options to the user, so let's set
cannot_instantiate_with_device_add_yet for them.

It's already set for Bonito, Grackle, i440FX and Raven.  Document why.

Set it for the others: dec-21154, e500-host-bridge, gt64120_pci, mch,
pbm-pci, ppc4xx-host-bridge, sh_pci_host, u3-agp, uni-north-agp,
uni-north-internal-pci, uni-north-pci, and versatile_pci_host.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>