iommu/vt-d: Set domain type for a private domain

Otherwise, domain_get_iommu() will be broken.

Fixes: 942067f1 ("iommu/vt-d: Identify default domains replaced with private")
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>