Two bug fixes, some cleanup, and a simplification for modern hardware.

1: properly reserve DID 0 for caching mode IOMMUs
2: split domid map cleanup check into a function
3: don't leak domid mapping on error path
4: tidy domid map handling
5: introduce helper to convert DID to domid_t
6: avoid allocating domid_{bit,}map[] when possible

This depends on the previously submitted (and already approved, except
for a possible release ack) "VT-d: per-domain IOMMU bitmap needs to
have dynamic size".

Jan