qemu: add capabilities bit for device ioh3420

qemu: add capabilities bit for device ioh3420

This is a PCIE "root port". It connects only to a port of the
integrated pcie.0 bus of a Q35 machine (can't be hotplugged), and
provides a single PCIe port that can have PCI or PCIe devices
hotplugged into it.

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