Mostly a note to self but others may find this snippet useful.
I use the following to install from a disk image stored in RBD. Make sure to fill in your own client username, secret UUID, and monitor addresses.
<disk type='network' device='cdrom'>
<driver name='qemu' type='raw'/>
<auth username='libvirt'>
<secret type='ceph' uuid='a487c228-159b-4197-8be9-8e0e0d2b8bd4'/>
</auth>
<source protocol='rbd' name='rados-pool/diskimage.iso'>
<host name='monitor-1.address' port='6789'/>
<host name='monitor-2.address' port='6789'/>
<host name='monitor-3.address' port='6789'/>
</source>
<target dev='hda' bus='ide'/>
<readonly/>
<alias name='ide0-0-0'/>
<address type='drive' controller='0' bus='0' target='0' unit='0'/>
</disk>