vfio/iommufd: Add comment emphasizing no movement of hiod->realize() call

vfio/iommufd: Add comment emphasizing no movement of hiod->realize() call

The nested IOMMU support needs device and hwpt id which are generated
only after attachment. Hiod encapsulates these information in realize()
and passes to vIOMMU.

Suggested-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250521110301.3313877-1-zhenzhong.duan@intel.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>