qemu: don't be as insistent about adding dmi-to-pci-bridge or pci-bridge

qemu: don't be as insistent about adding dmi-to-pci-bridge or pci-bridge

Previously there was no way to have a Q35 domain that didn't have
these two controllers. This patch skips their creation as long as
there are some other kinds of pci controllers at index 1 and 2
(e.g. some pcie-root-port controllers).

I'm hoping that soon we won't add them at all, plugging all devices
into auto-added pcie-*-port ports instead, but in the meantime this
makes it easier to experiment with alternative bus hierarchies.