bhyve: Make most I/O port handling specific to amd64

• The qemu_fwcfg interface, as implemented, is I/O port-based, but QEMU implements an MMIO interface that we'll eventually want to port for arm64.
• Retain support for I/O space PCI BARs, simply treat them like MMIO BARs for most purposes, similar to what the arm64 kernel does. Such BARs are created by virtio devices.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40741