intel-iommu: use realize instead of init in iommu_class_init()

TYPE_SYS_BUS_DEVICE should use #DeviceClass.realize instead. So change
vtd_init() to vtd_realize().

Signed-off-by: Le Tan <tamlokveer@gmail.com>
intel-iommu: change some iommu_ to vtd_

Rename iommu_class_init, iommu_info, iommu_register_types to vtd_class_init,
vtd_info and vtd_register_types.

Signed-off-by: Le Tan <tamlokveer@gmail.com>