iommufd/selftest: Prepare for mock_viommu_alloc_domain_nested()
A nested domain now can be allocated for a parent domain or for a vIOMMU
object. Rework the existing allocators to prepare for the latter case.
Link: https://patch.msgid.link/r/f62894ad8ccae28a8a616845947fe4b76135d79b.1730836219.git.nicolinc@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>