This patch reports nesting info when iommu_domain_get_attr() is called with
DOMAIN_ATTR_NESTING and one domain with nesting set.

v7 -> v8:
*) tweak per latest code base

v6 -> v7:
*) split the patch in v6 into two patches:
   [PATCH v7 15/16] iommu/vt-d: Only support nesting when nesting caps are consistent across iommu units
   [PATCH v7 16/16] iommu/vt-d: Support reporting nesting capability info

v2 -> v3:
*) remove cap/ecap_mask in iommu_nesting_info.

Cc: Kevin Tian <kevin.tian@intel.com>
CC: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Eric Auger <eric.auger@redhat.com>
Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>