iommufd/selftest: Add a helper to get test device

There is need to get the selftest device (sobj->type == TYPE_IDEV) in
multiple places, so have a helper to for it.

Link: https://patch.msgid.link/r/20250321171940.7213-17-yi.l.liu@intel.com


Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>