qemu: allow 32 slots on pcie-expander-bus, not just 1

qemu: allow 32 slots on pcie-expander-bus, not just 1

When I added support for the pcie-expander-bus controller in commit
bc07251f, I incorrectly thought that it only had a single slot
available. Actually it has 32 slots, just like the root complex aka
pcie-root (the part that I *did* get correct is that unlike pcie-root
a pcie-expander-bus doesn't allow any integrated endpoint devices -
only pcie-root-ports and dmi-to-pci-controllers are allowed).