iommu/vt-d: Mark copied context entries

Mark the context entries we copied over from the old kernel,
so that we don't detect them as present in other code paths.
This makes sure we safely overwrite old context entries when
a new domain is assigned.

Tested-by: ZhenHua Li <zhen-hual@hp.com>
Tested-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>