libvirt: configuration element for a random number generator device

This configuration exposes a paravirtual random number generator device,
with a default backend set to a hosts /dev/random

An optional rate element can be provided in order to limit the consumption
of host's entropy by the guest.

Partially implements blueprint libvirt-rng-device
Change-Id: Id1be32a307473aec2ff0b54b43b2ca3547d74b84
Merge "libvirt: configuration element for a random number generator device"