If bootverbose is enabled every vnet startup and virtual interface
creation will print extra lines on the console. We are generally not
interested in this (repeated) information for each VNET. Thus only
print it for the default VNET. Virtual interfaces on the base system
will remain printing information, but e.g. each loopback in each vnet
will no longer cause a "bpf attached" line.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: gnn
Differential Revision: https://reviews.freebsd.org/D4531