Qemu's implementation of virtio RNG supports rate limiting of the
entropy used. This patch exposes the option to tune this functionality.

This patch is based on qemu commit 904d6f588063fb5ad2b61998acdf1e73fb4

The rate limiting is exported in the XML as:
<devices>
  ...
  <rng model='virtio'>
    <rate bytes='123' period='1234'/>
    <backend model='random'/>
  </rng>
  ...