iommufd/selftest: Add a selftest for iommufd_device_attach() with a hwpt argument

This can now be covered since we have a full struct device.

Link: https://lore.kernel.org/r/12-v3-ae9c2975a131+2e1e8-iommufd_hwpt_jgg@nvidia.com


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