Creating bug from upstream F19 feature page:
Documentation for this feature would likely belong in the VGSG and the VDAG - assess whether this needs documenting for RHEL7.
VirtIO RNG (random number generator) is a paravirtualized device that is exposed as a hardware RNG device to the guest, to prevent entropy starvation in guests.
Para-virtualized devices are detailed in section 4.3.2. - a description of this device would likely fit best here: