qemu: add capabilities bit for device xio3130-downstream

qemu: add capabilities bit for device xio3130-downstream

The downstream ports of an x3130-upstream switch can each have one of
these plugged into them (and that is the only place they can be
connected). Each xio3130-downstream provides a single PCIe port that
can have PCI or PCIe devices hotplugged into it. Apparently an entire
set of x3130-upstream + several xio3130-downstreams can be hotplugged
as a unit, but it's not clear to me yet how that would be done, since
qemu only allows attaching a single device at a time.

This device will be used to implement the
"pcie-switch-downstream-port" model of pci controller.