conf: restrict what type of buses will accept a pci-bridge

conf: restrict what type of buses will accept a pci-bridge

A pci-bridge has *almost* the same rules as a legacy PCI endpoint
device for where it can be automatically connected, and until now both
had been considered identical. There is one pairing that is okay when
specifically requested by the user (i.e. manual assignment), but we
want to avoid it when auto-assigning addresses - plugging a pci-bridge
directly into pcie-root (it is cleaner to plug in a dmi-to-pci-bridge,
then plug the pci-bridge into that).

In order to allow that difference, this patch makes a separate
CONNECT_TYPE for pci-bridge, and uses it to restrict auto-assigned
addresses for pci-bridges to be only on pci-root, pci-expander-bus,
dmi-to-pci-bridge, or on another pci-bridge.

NB: As with other discouraged-but-seem-to-work configurations
(e.g. plugging a legacy PCI device into a pcie-root-port) if someone
*really* wants to, they can still force a pci-bridge to be plugged
into pcie-root (by manually specifying its PCI address.)