[libvirt] Enhancement: Random Clock Offset

For better system anonymity (to decouple VM OS timestamps leaked in traffic from host ones) a feature can be added to the clock offset variable to select randomly from a specified range of seconds from instead of a fixed number of seconds. That way a guest's clock can vary unpredictably from the host's and confuse correlation by network adversaries. Full Disclosure: I am from the Tor centric Whonix Project - whonix.org and this would be a very useful feature for us.