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.

* https://libvirt.org/formatdomain.html#elementsRng

Change-Id: I285f68c81b0587c6a197d4791b09234a1697f5e7
Closes-Bug: #1843542

openstack/nova,rahulunair/nova,klmitch/nova,mahak/nova,mahak/nova,klmitch/nova,openstack/nova,klmitch/nova,openstack/nova,mahak/nova,rahulunair/nova,rahulunair/nova,klmitch/nova