VT-d: drop ownership checking from domain_context_mapping_one()

VT-d: drop ownership checking from domain_context_mapping_one()

Despite putting in quite a bit of effort it was not possible to
establish why exactly this code exists (beyond possibly sanity
checking). Instead of a subsequent change further complicating this
logic, simply get rid of it.

Take the opportunity and move the respective unmap_vtd_domain_page() out
of the locked region.

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