We use only one device-mapper layer for HVMs, and this isn't the same as
for PV - it is that one, which PV does in initramfs.
Device-mapper layers summary for template-based VMs:
PV: root.img+root-cow.img (dom0) -> xvda, xvda+volatile.img (VM)
HVM: root.img+volatile.img (dom0)