fix(virtsecrets): the create dialog offers only the usage types it can create

The usage list of the create-secret dialog came from the map that names
the usage types of existing secrets, so it offered none and tls too; the
form takes ceph, volume and iscsi only, and picking none or tls always
failed with "Select a valid choice". The dialog now lists the form's own
choices.

virtsecrets/views.py

@@ -33,6 +33,8 @@ def secrets(request, compute_id):
         VIR_SECRET_USAGE_TYPE_ISCSI: "iscsi",
         VIR_SECRET_USAGE_TYPE_TLS: "tls",
     }
+    # the create dialog offers the usage types the form takes
+    usage_choices = AddSecret.base_fields["usage_type"].choices
 
     try:
         conn = wvmSecrets(