ACCESS_PLATFORM/ORDER_PLATFORM

This is an attempt to clarify the intent behind
VIRTIO_F_IOMMU_PLATFORM and VIRTIO_F_IO_BARRIER
which based on recent discussions appear to be hard to understand.

- rename VIRTIO_F_IOMMU_PLATFORM to ACCESS_PLATFORM
  It is already the fact that the DMA API that Linux
  uses does more than just IOMMUs - it includes
  cache flushing, bounce buffers for limited
  addressing, etc.
  Update spec to match this reality.

- rename VIRTIO_F_IO_BARRIER to VIRTIO_F_ORDER_PLATFORM
  this is after all what device is telling driver:
  its memory accesses are only ordered weakly,
  this is why a stronger barrier is required.

- As no one yet implemented IO_BARRIER yet, add a recommendation
  to have a software fallback so that existing drivers
  aren't broken.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/25
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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