A working example of a Ceph RBD pool for VMM

<pool type="rbd">
  <name>RBD</name>
  <source>
    <host name="192.168.7.23" port="6789"/>
    <host name="192.168.7.31" port="6789"/>
    <name>rbd</name>
    <auth type="ceph" username="libvirt">
      <secret uuid="ce81a9d5-e184-43f5-9025-9a062d595fcb"/>
    </auth>
  </source>
</pool>

Host elements are addresses to mons. Auth element is a pointer to a registered key. On this webpage you can find information about how to register a Ceph key. https://docs.ceph.com/en/latest/rbd/libvirt/ The first name element is the name for the libvirt pool. The second name is the name of the Ceph RBD pool. In Connection manager in Virtual Machine Manager you have to create a pool and paste the XML into the textbox on the XML tab.