virtio_mmio: Support non-transitional version 2 devices

The non-legacy virtio MMIO specification drops the use of PFNs and
replaces them with physical addresses. Whilst many implementations are
so-called transitional devices, also implementing the legacy
specification, TinyEMU[1] does not. Device-specific configuration
registers have also changed to being little-endian, and must be accessed
using a single aligned access for registers up to 32 bits, and two
32-bit aligned accesses for 64-bit registers.

[1] https://bellard.org/tinyemu/

Reviewed by: br, brooks (mentor)
Approved by: br, brooks (mentor)
Differential Revision: https://reviews.freebsd.org/D24681