iommufd/selftest: Add coverage for IOMMU_HWPT_ALLOC with nested HWPTs

The IOMMU_HWPT_ALLOC ioctl now supports passing user_data to allocate a
user-managed domain for nested HWPTs. Add its coverage for that. Also,
update _test_cmd_hwpt_alloc() and add test_cmd/err_hwpt_alloc_nested().

Link: https://lore.kernel.org/r/20231026043938.63898-11-yi.l.liu@intel.com


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