For unmanaged ethernet <interface/>, it is user's responsibility
to set up the interface. And as such it can be just anything.
Therefore, it's (almost) impossible for the
virDomainInterfaceStats() API to tell whether RX/TX values need
to be swapped or copied verbatim into the return structure.
Document this limitation.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>