Starting the debugging.

First, we must have qemu-block-extra.  After adding that we can check that qemu itself supports rbd with:

qemu-system-x86_64 -drive format=? 2>&1 | grep rbd

Next, we see libvirt barfing on the missing block device it tried to add and spits out the disk it was going to use:

2015-09-16 14:31:45.853+0000: 7856: error : qemuMonitorJSONCheckError:381 : internal error: unable to execute QEMU command 'device_add': Property 'virtio-blk-device.drive' can't find value 'drive-virtio-disk1'
2015-09-16 14:31:45.857+0000: 7856: error : qemuMonitorTextDriveDel:2599 : operation failed: deleting file=rbd:cinder-ceph/volume-da89c042-0628-4630-9d95-4624452d346c:id=nova-compute:key=AQDUdflVYhZ4FhAAmT/7O1S3bUpOHaAFBqq1SA==:auth_supported=cephx\;none:mon_host=10.5.29.172\:6789\;10.5.29.173\:6789\;10.5.29.174\:6789,if=none,id=drive-virtio-disk1,format=raw,serial=da89c042-0628-4630-9d95-4624452d346c,cache=none drive failed: 2015-09-16T14:31:45.857818Z Device 'file=rbd:cinder-ceph/volume-da89c042-0628-4630-9d95-4624452d346c:id=nova-compute:key=AQDUdflVYhZ4FhAAmT/7O1S3bUpOHaAFBqq1SA==:auth_supported=cephx\;none:mon_host=10.5.29.172\:6789\;10.5.29.173\:6789\;10.5.29.174\:6789,if=none,id=drive-virtio-disk1,format=raw,serial=da89c042-0628-4630-9d95-4624452d346c,cache=none' not found^M

Attempted to run qemu (now with rbd support) with the above -drive line:

# qemu-system-x86_64 -monitor stdio -S -nographic -drive file=rbd:cinder-ceph/volume-da89c042-0628-4630-9d95-4624452d346c:id=nova-compute:key=AQDUdflVYhZ4FhAAmT/7O
1S3bUpOHaAFBqq1SA==:auth_supported=cephx\;none:mon_host=10.5.29.172\:6789\;10.5.29.173\:6789\;10.5.29.174\:6789,if=none,id=drive-virtio-disk1,format=raw,serial=da89c042-0628-4630-9d95-4624452d346c,cache=none     
qemu-system-x86_64: -drive file=rbd:cinder-ceph/volume-da89c042-0628-4630-9d95-4624452d346c:id=nova-compute:key=AQDUdflVYhZ4FhAAmT/7O1S3bUpOHaAFBqq1SA==:auth_supported=cephx;none:mon_host=10.5.29.172:6789;10.5.29
.173:6789;10.5.29.174:6789,if=none,id=drive-virtio-disk1,format=raw,serial=da89c042-0628-4630-9d95-4624452d346c,cache=none: conf option 6789;10.5.29.173:6789;10.5.29.174:6789 has no value
root@juju-devel3-machine-14:/var/log/libvirt/qemu# qemu-system-x86_64 -monitor stdio -S -nographic -drive file=rbd:cinder-ceph/volume-da89c042-0628-4630-9d95-4624452d346c:id=nova-compute:key=AQDUdflVYhZ4FhAAmT/7O
1S3bUpOHaAFBqq1SA==:auth_supported=cephx;none:mon_host=10.5.29.172:6789;10.5.29.173:6789;10.5.29.174:6789;,if=none,id=drive-virtio-disk1,format=raw,serial=da89c042-0628-4630-9d95-4624452d346c,cache=none          
WARNING: Image format was not specified for 'rbd:cinder-ceph/volume-da89c042-0628-4630-9d95-4624452d346c:id=nova-compute:key=AQDUdflVYhZ4FhAAmT/7O1S3bUpOHaAFBqq1SA==:auth_supported=cephx' and probing guessed raw.
         Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
         Specify the 'raw' format explicitly to remove the restrictions.
QEMU 2.3.0 monitor - type 'help' for more information
(qemu) qemu-system-x86_64: cannot use stdio by multiple character devices
none:mon_host=10.5.29.172:6789: command not found
10.5.29.173:6789: command not found
10.5.29.174:6789: command not found
,if=none,id=drive-virtio-disk1,format=raw,serial=da89c042-0628-4630-9d95-4624452d346c,cache=none: command not found

After dropping none: through all the mon_host options, I can connect.   Something in the option passing/parsing isn't quite right.  diging.