iommufd/selftest: Rename domain_id to stdev_id for FIXTURE iommufd_ioas

Nothing uses this for anything more than checking if a mock domain had
been created. Rename it to stdev_id to match our naming system. Currently
domain_id is the hwpt_id.

Link: https://lore.kernel.org/r/8-v3-ae9c2975a131+2e1e8-iommufd_hwpt_jgg@nvidia.com


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