iommu/vt-d: Delegate the identity domain to upper layer

This allows the iommu generic layer to allocate an identity domain
and attach it to a device. Hence, the identity domain is delegated
to upper layer. As a side effect, iommu_identity_mapping can't be
used to check the existence of identity domains any more.

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