Support MMIO devices in VFIO
Additional information:
- https://lore.kernel.org/qemu-devel/cover.1667542066.git.john.g.johnson@oracle.com/
- https://github.com/nutanix/libvfio-user
- It also *somewhat* related to supporting non-PCI devices in `ivshmem`: https://gitlab.com/qemu-project/qemu/-/issues/1134