ixl: Tie statistics baselines to hardware resource epochs

[kbowling]

Summary

Raw VSI statistics belong to the firmware-assigned counter index and are
reset when firmware recreates the VSI.  Reusing a baseline across either
event makes a counter decrease look like a full-width rollover.

Reset the PF and main-VSI baselines when rebuilding hardware resources.
Start a new baseline when firmware assigns a different PF counter index
and whenever it allocates a VF VSI.  Ordinary interface reinits which
retain the hardware resources continue to retain their statistics.

This follows the lifecycle used by ice without placing an unconditional
statistics reset in ixl_initialize_vsi(), which also runs during ordinary
iflib initialization.

MFC after:      2 weeks
Sponsored by:   BBOX.io