iommu/vt-d: Add for_each_device_domain() helper

This adds a helper named for_each_device_domain() to iterate
over the elements in device_domain_list and invoke a callback
against each element. This allows to search the device_domain
list in other source files.

Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Liu Yi L <yi.l.liu@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Liu Yi L <yi.l.liu@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>