enable openqa disk image generation

the servers have nested virt enabled now, so this should be OK

roles/openqa/server/tasks/main.yml

@@ -97,11 +97,7 @@
     chdir: /var/lib/openqa/share/factory/hdd/
   environment:
     LIBGUESTFS_BACKEND: direct
-  # This is disabled until we enable nested virt on the server VMs;
-  # image creation involves running VMs and takes *forever* without
-  # KVM acceleration. Once we turn it on, the condition should be
-  # "diskcheck.rc > 0".
-  when: "1 != 1"
+  when: "diskcheck.rc > 0"
 
 - name: Create exports file
   copy: src=exports dest=/etc/exports.d/openqa.exports owner=root group=root mode=0644