iommufd/selftest: Replace mock_viommu_alloc with mock_viommu_init

iommufd/selftest: Replace mock_viommu_alloc with mock_viommu_init

To ease the for-driver iommufd APIs, get_viommu_size and viommu_init ops
are introduced.

Sanitize the inputs and report the size of struct mock_viommu on success,
in mock_get_viommu_size().

The core will ensure the viommu_type is set to the core vIOMMU object, so
simply init the driver part in mock_viommu_init().

Remove the mock_viommu_alloc, completing the replacement.

Link: https://patch.msgid.link/r/993beabbb0bc9705d979a92801ea5ed5996a34eb.1749882255.git.nicolinc@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>