libvirt reporting spice not supported in 2.3.1

I am running Ubuntu 15.04 (not 15.10/dev). Due to another bug in qemu 2.2 I decided to try the 2.3.1 packages (from build here). I realise there is potential dependency issues and the config is likely untested.

However as this could be a genuin issue, I wanted to report that With these installed, along with latest libvirt I noticed that whilst the bug I needed a fix to was addressed (a savevm issue), any VM configured to use "spice" would not launch.

libvirt (via virt-manager) reports
"Error starting domain: unsupported configuration: spice graphics are not supported with this QEMU"
which is also reflected in syslog:
Jul 8 11:50:36 jonesn-ThinkPad-W530 libvirtd[2142]: unsupported configuration: spice graphics are not supported with this QEMU

On reviewing the build logs, it appears qemu HAS been built with spice ie it reports:
spice support yes (0.12.7/0.12.5)

As some additional validation I also then replaced these packages with another private ppa (a second opinion) - in no means official of course. https://launchpad.net/~jacob/+archive/ubuntu/virtualisation . These packages, which also include virt-manager also result in the same error

It's possible that spice support is being incorrectly validated

Relevant extracts of my VM definition include

   <emulator>/usr/bin/kvm-spice</emulator>

    <channel type='spicevmc'>
      <target type='virtio' name='com.redhat.spice.0'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    </channel>

    <graphics type='spice' autoport='yes'/>

   <video>
      <model type='qxl' ram='65536' vram='65536' vgamem='16384' heads='1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>