iommufd/selftest: Rename the sefltest 'device_id' to 'stdev_id'

It is too confusing now that we have the 'dev_id' as part of the main
interface. Make it clear this is the special selftest device object. This
object is analogous to the VFIO device FD.

Link: https://lore.kernel.org/r/7-v3-ae9c2975a131+2e1e8-iommufd_hwpt_jgg@nvidia.com


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