Convert vmm_ops calls to IFUNC

There is no need for these to be function pointers since they are
never modified post-module load.

Rename AMD/Intel ops to be more consistent.

Submitted by: adam_fenn.io
Reviewed by: markj, grehan
Approved by: grehan (bhyve)
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D27375