iommufd/selftest: Add nested domain allocation for mock domain

Add nested domain support in the ->domain_alloc_user op with some proper
sanity checks. Then, add a domain_nested_ops for all nested domains and
split the get_md_pagetable helper into paging and nested helpers.

Also, add an iotlb as a testing property of a nested domain.

Link: https://lore.kernel.org/r/20231026043938.63898-10-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>