nova-manage: Add libvirt list_unset_machine_type command

This change adds a libvirt command to list all instance UUIDs with
hw_machine_type unset in their image metadata. This will be useful to
operators attempting to change the [libvirt]hw_machine_type default in
the future as it allows them to confirm if it is safe to change the
configurable without impacting existing instances.

blueprint: libvirt-default-machine-type
Change-Id: I39909ace97f62e87f326d4d822d4e4c391445765

doc/source/cli/nova-manage.rst

@@ -775,6 +775,30 @@ libvirt
        * - 5
          - The provided machine type is unsupported
 
+``nova-manage libvirt list_unset_machine_type [--cell-uuid]``
+    List the UUID of any instance without ``hw_machine_type`` set.
+
+    This command is useful for operators attempting to determine when it is
+    safe to change the :oslo.config:option:`libvirt.hw_machine_type` option
+    within an environment.
+
+    **Return Codes**
+
+    .. list-table::
+       :widths: 20 80
+       :header-rows: 1
+
+       * - Return code
+         - Description
+       * - 0
+         - Completed successfully, no instances found without hw_machine_type
+       * - 1
+         - An unexpected error occurred
+       * - 2
+         - Unable to find cell mapping
+       * - 3
+         - Instances found without hw_machine_type set
+
 See Also
 ========