amd iommu: disable iommu emulation on non-iommu systems

amd iommu: disable iommu emulation on non-iommu systems

Introduce a new flag to disable iommu emulation on old iommu systems.
This patch is taken from my v4 patch queue, which is till pending, to
make old or non-iommu system to run cleanly without interfered by
iommuv2 codes. This might be helpful to isolate iommuv2 code in
debugging unstable regressions. The reset part of v4 will be re-based.

Signed-off-by: Wei Wang <wei.wang2@amd.com>
Committed-by: Keir Fraser <keir@xen.org>