VT-d: avoid allocating domid_{bit,}map[] when possible

VT-d: avoid allocating domid_{bit,}map[] when possible

When an IOMMU implements the full 16 bits worth of DID in context
entries, there's no point going through a memory base translation table.
For IOMMUs not using Caching Mode we can simply use the domain IDs
verbatim, while for Caching Mode we need to avoid DID 0.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>