iommufd/selftest: Add IOMMU_VDEVICE_ALLOC test coverage

iommufd/selftest: Add IOMMU_VDEVICE_ALLOC test coverage

Add a vdevice_alloc op to the viommu mock_viommu_ops for the coverage of
IOMMU_VIOMMU_TYPE_SELFTEST allocations. Then, add a vdevice_alloc TEST_F
to cover the IOMMU_VDEVICE_ALLOC ioctl.

Link: https://patch.msgid.link/r/4b9607e5b86726c8baa7b89bd48123fb44104a23.1730836308.git.nicolinc@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>