libvirt: remove explicit /dev/random rng default

libvirt (or rather qemu) will default to /dev/random if no rng device
path is specified i.e.

  <rng model='virtio'>
    <backend model='random'/>
  </rng>

It's preferable for us to not duplicate this default to allow for a
future where libvirt or the hypervisor needs to make more intelligent
decisions about the default device to use.

Also, improve the unit tests to catch this issue.

Change-Id: Ia0f922f7de575496972b4dfa9d56aa22205f1758
Merge "libvirt: remove explicit /dev/random rng default"