virNetDevTapInterfaceStats: Allow caller to not swap the statistics

virNetDevTapInterfaceStats: Allow caller to not swap the statistics

https://bugzilla.redhat.com/show_bug.cgi?id=1497410

The comment in virNetDevTapInterfaceStats() implementation for
Linux states that packets transmitted by domain are received by
the host and vice versa. Well, this is true but not for all types
of interfaces. For instance, for macvtaps when TAP device is
hooked right onto a physical device any packet that domain sends
looks also like a packet sent to the host. Therefore, we should
allow caller to chose if the stats returned should be straight
copy or swapped.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>