IOMMU/x86: maintain a per-device pseudo domain ID

IOMMU/x86: maintain a per-device pseudo domain ID

In order to subsequently enable per-device quarantine page tables, we'll
need domain-ID-like identifiers to be inserted in the respective device
(AMD) or context (Intel) table entries alongside the per-device page
table root addresses.

Make use of "real" domain IDs occupying only half of the value range
coverable by domid_t.

Note that in VT-d's iommu_alloc() I didn't want to introduce new memory
leaks in case of error, but existing ones don't get plugged - that'll be
the subject of a later change.

The VT-d changes are slightly asymmetric, but this way we can avoid
assigning pseudo domain IDs to devices which would never be mapped while
still avoiding to add a new parameter to domain_context_unmap().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>