Fix unit of hw_rng:rate_period
Fix unit of hw_rng:rate_period

Replace seconds with milliseconds
in the description of hw_rng:rate_period[1].

[1]https://libvirt.org/formatdomain.html#elementsRng

Change-Id: I53848359704d68dd84c5be8106f4e259937b8092
Related-Bug:#1843542

openstack/glance,openstack/glance,openstack/glance