More misunderstanding/mistaken assumptions on my part - I had thought
that a pci-expander-bus could be plugged into any legacy PCI slot, and
that pcie-expander-bus could be plugged into any PCIe slot. This isn't
correct - they can both be plugged ontly into their respective root
buses. This patch adds that restriction.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1358712