iommufd/selftest: Rename domain_id to hwpt_id for FIXTURE iommufd_mock_domain

In this case the domain_id was acting as the hwpt_id, so be clearer.

Link: https://lore.kernel.org/r/9-v3-ae9c2975a131+2e1e8-iommufd_hwpt_jgg@nvidia.com


Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>