vfio/cpr-iommufd.c: use QOM casts where appropriate

Use QOM casts to convert between VFIOIOMMUFDContainer and VFIOContainer instead
of accessing bcontainer directly.

Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250925113159.1760317-9-mark.caveayland@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/vfio/cpr-iommufd.c

@@ -176,7 +176,7 @@ void vfio_iommufd_cpr_unregister_iommufd(IOMMUFDBackend *be)
 bool vfio_iommufd_cpr_register_container(VFIOIOMMUFDContainer *container,
                                          Error **errp)
 {
-    VFIOContainer *bcontainer = &container->bcontainer;
+    VFIOContainer *bcontainer = VFIO_IOMMU(container);
 
     migration_add_notifier_mode(&bcontainer->cpr_reboot_notifier,
                                 vfio_cpr_reboot_notifier,
@@ -189,7 +189,7 @@ bool vfio_iommufd_cpr_register_container(VFIOIOMMUFDContainer *container,
 
 void vfio_iommufd_cpr_unregister_container(VFIOIOMMUFDContainer *container)
 {
-    VFIOContainer *bcontainer = &container->bcontainer;
+    VFIOContainer *bcontainer = VFIO_IOMMU(container);
 
     migration_remove_notifier(&bcontainer->cpr_reboot_notifier);
 }