block/iscsi: use sector_limits_lun2qemu throughout iscsi_refresh_limits

As Max pointed out there is a hidden cast from int64_t to int for all
limits. So use the newly introduced sector_limits_lun2qemu for all
limits received from the target.

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
