VT-d: introduce helper to convert DID to domid_t

VT-d: introduce helper to convert DID to domid_t

This is in preparation of adding another "translation" method. Take the
combination of the extra validation both previously open-coded have been
doing: Bounds check and bitmap check. But don't propagate the previous
pointless check of whether ->domid_map[] was actually allocated, as
failure there would lead to overall failure of IOMMU initialization
anyway.

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