VT-d: don't needlessly look up DID

VT-d: don't needlessly look up DID

If get_iommu_domid() in domain_context_unmap_one() fails, we better
wouldn't clear the context entry in the first place, as we're then unable
to issue the corresponding flush. However, we have no need to look up the
DID in the first place: What needs flushing is very specifically the DID
that was in the context entry before our clearing of it.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>