Fix detection of all absent VMs

`rejectattr` fails if the attribute is not set, and no default option is
available. Alter the logic to ensure that if `state` is unset, it is
treated as `present`.

tasks/main.yml

@@ -1,7 +1,10 @@
 ---
 - include_tasks: autodetect.yml
-  # We need to know the engine and emulator if we're creating any new VMs.
-  when: libvirt_vms | rejectattr('state', 'eq', 'absent') | list
+  # We don't need to know the engine and emulator if we're not creating any new
+  # VMs.
+  when: >-
+    (libvirt_vms | selectattr('state', 'defined')
+       | selectattr('state', 'eq', 'absent') | list) != libvirt_vms
 
 - include_tasks: volumes.yml
   vars: