sfxge: add prefast annotation to common code return types

Using a typedef for common code return types (rather than "int")
allows the Prefast static analyser to understand when a function
has been successful (and thus when its postconditions must hold).

This greatly reduces then number of false positives reported by
prefast for error paths in common code functions.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days