conf: Reword error message to be more universal

conf: Reword error message to be more universal

The error message reported when attempting to change/get persistent
configuration of a transient domain suggests that changes are being
made. Reword it to suit getter APIs too.

Before:
$ virsh vcpucount transient-domain --config
error: Requested operation is not valid: cannot change persistent config of a transient domain

After:
$ virsh vcpucount transient-domain --config
error: Requested operation is not valid: transient domains do not have any persistent config