testing: Use host's /dev/urandom as /dev/random on guests via VirtIO RNG

Newer versions of systemd etc. seem to require quite a lot of entropy
from /dev/random while booting, which can block and therefore delay the
start of other services (in particular sshd) by more than a minute.
Using the host's /dev/urandom via VirtIO RNG, we can avoid blocking the
guests.

The required kernel options are added for kernel versions 5.4+.

testing/config/kvm/moon.xml

@@ -73,5 +73,8 @@
     <memballoon model='virtio'>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
     </memballoon>
+    <rng model='virtio'>
+      <backend model='random'>/dev/urandom</backend>
+    </rng>
   </devices>
 </domain>