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 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.