This is backed by the qemu device "pxb".

The pxb device always includes a pci-bridge that is at the bus number
of the pxb + 1.

busNr and <node> from the <target> subelement are used to set the
bus_nr and numa_node options for pxb.

During post-parse we validate that the domain's machinetype is
440fx-based (since the pxb device only works on 440fx-based machines),
and <node> also gets a sanity check to assure that the NUMA node
specified for the pxb (if any - it's optional) actually exists on the
guest.