libvmmapi: Conditionalize compilation of some functions

Hide definitions of several functions that currently don't have
implementatations in the arm64 vmm port. In particular, add a
WITH_VMMAPI_SNAPSHOT preprocessor variable that can be used to enable
compilation of save/restore functions, and conditionalize compilation of
some functions only used by amd64 bhyve. If in the long term they
remain amd64-only, they can move to vmmapi_machdep.c, but for now it's
not clear to me that that's the right thing to do.

MFC after: 2 weeks
Sponsored by: Innovate UK