With a similar configuration, this is the error I get from virsh when composing the VM from MAAS:
provisioningserver.drivers.pod.virsh.VirshError: Virsh command ['vol-create-as', 'loop29', '15f71b1f-76b3-47f7-adbb-d87852d22f77', '1000000000', '--allocation', '0', '--format', 'raw'] failed: Failed to create vol 15f71b1f-76b3-47f7-adbb-d87852d22f77 error: unsupported configuration: unknown volume format type raw
With a similar configuration, this is the error I get from virsh when composing the VM from MAAS:
error: unsupported configuration: unknown volume format type raw