ioh3420 is emulated Intel hardware, so it always looked
quite out of place in aarch64/virt guests. Even for x86/q35
guests, the recently-introduced pcie-root-port is a better
choice because, unlike ioh3420, it doesn't require IO space
(a fairly constrained resource) to work.

If pcie-root-port is available in QEMU, use it; ioh3420 is
still used as fallback for when pcie-root-port is not
available.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1408808