[virtio] Wire BTIs through to the device objects

Change-Id: I8a19733734b8694f734463dd3134b77a9ae50d14

[virtio] Refactor virtio backend support

- Implement device backends for transport rather than having
it be a part of the device class itself. These backends are
plugged into the device class to provide an abstraction between
device level interfaces and the specific nuances around the
virtio interface.
- Implement a legacy virtio bar0 PIO interface for use in systems
that only provide Virtio 0.9.5 devices.
- Move logging away from VIRTIO_ERROR and use newer ddk zxlogf.
- Run clang-fmt and clean up small nits here and there.
Change-Id: I2568e9554378163f19b89537986e0d5ea8b47510

[virtio] Add a virtio-input driver.

Currently only supports the QEMU virual keyboard and not very well.

Change-Id: Ia80d9a6f601c85ca408d5aa30196a3b69886b665