pmap: Implement pmap_npt_transform() for NVMM

pmap: Implement pmap_npt_transform() for NVMM

This function will transform an initialized pmap structure for use by
NVMM's AMD SVM backend.

AMD's NPT (nested page table), aka RVI (rapid virtualization indexing)
implementation is more complete than Intel's EPT; it supports A/D bits
and uses the same bits positions as native x86 page tables.  So this
function is a simplified version of pmap_ept_transform().