Since libvirt 1.3.4, any RNG source is accepted for the 'random'
backend.  However, '/dev/urandom' is the _recommended_ source of
entropy. Therefore we should mention that in the docs.
Suggested-by: NDaniel P. Berrangé <berrange@redhat.com>
Signed-off-by: NKashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>