core/storage: fix disk handling for HVM template
core/storage: fix disk handling for HVM template

Currently HVM template do not have root-cow.img (also do not use 2-layer
device-mapper as PV VMs), so vm.is_template() check isn't enough.

QubesOS/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin