iommufd/selftest: Rename the remaining mock device_id's to stdev_id

For consistency.

Link: https://lore.kernel.org/r/10-v3-ae9c2975a131+2e1e8-iommufd_hwpt_jgg@nvidia.com


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