Deve ser tão fácil quanto virsh edit DOMAIN
e alterar o atributo vram
de /domain/devices/video/model
:
...
<video>
<model type='cirrus' vram='9216' heads='1'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
...