intel-iommu: Unify hardware and software passthrough support

This makes the hardware passthrough mode work a lot more like the
software version, so that the behaviour of a kernel with 'iommu=pt'
is the same whether the hardware supports passthrough or not.

In particular:
 - We use a single si_domain for the pass-through devices.
 - 32-bit devices can be taken out of the pass-through domain so that
   they don't have to use swiotlb.
 - Devices will work again after being removed from a KVM guest.
 - A potential oops on OOM (in init_context_pass_through()) is fixed.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>