MFC @ r256071

This is the change where the bhyve_npt_pmap branch was
merged in to head.

The SVM changes to work with this will be in a follow-on
submit.
Enable memory overcommit for AMD processors.

 - No emulation of A/D bits is required since AMD-V RVI
supports A/D bits.
 - Enable pmap PT_RVI support(w/o PAT) which is required for
memory over-commit support.
 - Other minor fixes:
 * Make use of VMCB EXITINTINFO field. If a #VMEXIT happens while
delivering an interrupt, EXITINTINFO has all the details that bhyve
needs to inject the same interrupt.
 * SVM h/w decode assist code was incomplete - removed for now.
 * Some minor code clean-up (more coming).

Submitted by:	Anish Gupta (akgupt3@gmail.com)