vtnet: Prefer "hardware" accounting for the multicast and total number of octets sent
When ALTQ is enabled, this driver does "hardware" accounting and soft
accounting at the same time. Prefer the "hardware" one to make the logic
simpler.
Reviewed by: zlei
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D44817
(cherry picked from commit 2a346c8993cbb92a321a7c25bd9ac4dcaae352d1)