iommufd/selftest: Add coverage for IOMMU_GET_HW_INFO ioctl

Add a mock_domain_hw_info function and an iommu_test_hw_info data
structure. This allows to test the IOMMU_GET_HW_INFO ioctl passing the
test_reg value for the mock_dev.

Link: https://lore.kernel.org/r/20230818101033.4100-5-yi.l.liu@intel.com


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