iommu: Add is_attach_deferred call-back to iommu-ops

This new call-back will be used to check if the domain attach need be
deferred for now. If yes, the domain attach/detach will return directly.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>