intel-iommu: set iommu_superpage on VM domains to lowest common denominator

set dmar->iommu_superpage field to the smallest common denominator
of super page sizes supported by all active VT-d engines.  Initialize
this field in intel_iommu_domain_init() API so intel_iommu_map() API
will be able to use iommu_superpage field to determine the appropriate
super page size to use.

Signed-off-by: Allen Kay <allen.m.kay@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>