Add the domain_alloc_user() op implementation. It supports allocating
domains to be used as parent under nested translation.

Unlike other drivers VT-D uses only a single page table format so it only
needs to check if the HW can support nesting.

Link: https://lore.kernel.org/r/20230928071528.26258-7-yi.l.liu@intel.comSigned-off-by: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>