sfxge(4): cleanup: make VPD lookups quieter

A lookup on a VPD entry which is missing reports several failure
messages as it propagates through wrapper functions. Restructured
the wrappers to treat this gracefully as an expected case.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6366