iommu/vt-d: Add get_domain_info() helper

Add a get_domain_info() helper to retrieve the valid per-device
iommu private data.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20200516062101.29541-10-baolu.lu@linux.intel.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>