virtio: Support MMIO bus for all devices

[jrtc27]

Summary

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.

[br]

we should also add these devices to riscv kernel config (at least random and console)

[jrtc27]

In D24730#544269, @br wrote:

we should also add these devices to riscv kernel config (at least random and console)

None of the other GENERIC configs list them. I assume they normally get picked up as modules? We can either do that or, if we really want them linked in to the kernel, modify our own kernel config (which we already need to change the kernel's LMA and tell it to mount /dev/vtbd0 as the root filesystem). But I don't think our GENERIC config should diverge in this regard.