iavf(4): Get MSI-X BAR index at runtime instead of hardcoding it

This allows iavf to load on E830 devices since those devices place their MSI-X
BAR at a different location than in previous 800 series products.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D46952

(cherry picked from commit e53a21abdf2953714e44e3c54b4bb78557cb096c)