vtnet: Do "hardware" accounting for the total number of received octets

While here, advertise the IFCAP_HWSTATS capability to avoid the net
stack from double counting it.

Co-authored-by: zlei
Reviewed by: zlei
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D44816

(cherry picked from commit a14d561e58529c9686a2efc47f4828ad82026e63)