[...]
I think you're right. Each bus requires some amount of IO space, and I
thought I recalled someone saying that all of the available IO space is
exhausted after 7 or 8 buses. This was in relation to PCIe, where each
root port is a bus, and can potentially take up IO space, so possibly in
that context they were talking about the buses *after* the root bus and
pcie-pci-bridge, which would bring us back to the same number you're
getting.

For PCIe our solution was to turn off IO space usage on the
pcie-root-ports, but you can't do that for conventional PCI buses, since
the devices might actually need IO space to function properly and the
standard requires that you provide it.

> The real question though is why you need to create sooooo many PCI buses.
> Each bus can do 31 devices.  Do you really need to have more than 279
> devices in your VM ?

And if you do need more than 279 devices, do they all need to be
hot-pluggable? If not, then you can put up to 8 devices on each slot
(functions 0 - 7).