iommu/vt-d: Consolidate domain_init() to avoid duplication

The domain_init() and md_domain_init() do almost the same job.
Consolidate them to avoid duplication.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>