[WIP]
This callback is related to Hyper-V Enlightenments. It is notified in function HvlPhase2Initialize only if bit 0 of HvlpRootFlags is not set. We still need to trace down wich Enlightenment set or not this bit, it looks like this is done in function HvlpDetermineEnlightenments which comes from HvlPhase0Initialize.
This still needs a lot of investigation since we are not even near to understand how Hyper-V works internally.