The pxb device is a PCIe expander bus that can be added to any
Q35-based machinetype. A single PCIe port (*not* hotpluggable) is
provided; if more than one device is desired, or if hotplug
support is needed, either a pcie-root-port, or some combination of
pcie-switch-upstream-port and pcie-swith-downstream-ports must be
added to it. It can have a NUMA node number associated with it, as
well as a bus number.