Merge the following changes from projects/bhyve_npt_pmap:
- add fields to 'struct pmap' that are required to manage nested page tables.
- add a parameter to 'vmspace_alloc()' that can be used to override the
  default pmap initialization routine 'pmap_pinit()'.

These changes are pushed ahead of the remaining changes in 'bhyve_npt_pmap'
in anticipation of the upcoming KBI freeze for 10.0.

Reviewed by:	kib@, alc@
Approved by:	re (glebius)