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The variables iommu_enabled and iommu_dont_flush_iotlb are defined in
drivers/passthrough/iommu.c and are referenced in common code, which
causes the link to fail when !CONFIG_HAS_PASSTHROUGH.

Guard references to these variables in common code so that xen
builds when !CONFIG_HAS_PASSTHROUGH.
Signed-off-by: Connor Davis <connojdavis@gmail.com>
[jb: further massage xen/iommu.h adjustment]
Acked-by: Jan Beulich <jbeulich@suse.com>