libvirt VM using video device with 2 heads not working

Question

I'm trying to set up a libvirt VM and I've got a VM which is supposed to have 2 screens. I've set video in the libvirt XML file to use 2 heads but I only get one screen for some reason.

I can see there are 4 displays defined in the machine, but only 1 is connected and I can't seem to get the others to connect. In the Virt Viewer View->Displays menu only Display-1 is selectable, the remaining screens are grayed out.

Excerpt from my XML file:

<video>
  <model type='qxl' vram='16384' heads='2'/>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>

xrandr output on my VM:

Screen 0: minimum 320 x 200, current 1024 x 768, maximum 8192 x 8192
Virtual-0 connected 1024x768+0+0 0mm x 0mm
    1024x768     59.92*+
    1920x1200    59.88
...<cut out the rest of the modes for brevity>...
Virtual-1 disconnected
Virtual-2 disconnected
Virtual-3 disconnected

Obviously I'm missing something here, but I can't seem to figure out what it is. Any help would be much appreciated!