[dev][virtio] Move interrupt syscalls into backend

This will make it much easier to test virtio drivers by implementing a
testing backend that overrides these functions to avoid syscalls.

Change-Id: I926fbd4434b50021d10d35d0bb831d2833efabaf

[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