virtio: Support MMIO bus for all devices

The bus is independent of the device, so all devices can be attached to
either a PCI bus or an MMIO bus. For example, QEMU's virtio-rng-device
gives the MMIO variant of virtio-rng-pci, and is now detected.

Reviewed by: andrew, br, brooks (mentor)
Approved by: andrew, br, brooks (mentor)
Differential Revision: https://reviews.freebsd.org/D24730