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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. Thanks, Wei Signed-off-by: Wei Wang