block/throttle-groups: fix throttle properties typename
The getter and setter use visit_type_int64, not visit_type_int.
Fixes: 432d889e55e2 ("block: convert ThrottleGroup to object with QOM")
Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>