doc: schema: Add basic documentation for the virtual RNG device support

doc: schema: Add basic documentation for the virtual RNG device support

This patch documents XML elements used for (basic) support of virtual
RNG devices.

In the devices section in the domain XML users may specify:

For the default 'random' backend:
  <devices>
    <rng model='virtio'>
      <backend model='random'>/dev/urandom</backend>
    </rng>
  </devices>

For the slightly more advanced EGD backend:
  <devices>
    <rng model='virtio'>
      <backend model='egd' type='udp'>
        <!-- this is a definition of a character device -->
        <source mode='bind' service='1234'/>
        <source mode='connect' host='1.2.3.4' service='1234'/>
        <!-- or other valid character device configuration -->
      </backend>
    </rng>
  </devices>

For the planned random daemon/pool:
  <devices>
    <rng model='virtio'>
      <backend model='pool' pool='poolname'>class</backend>
    </rng>
  </devices>

to enable the RNG device for guests.