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

libvmmapi: Move PCI passthrough ioctl wrappers into a separate file

The arm64 port doesn't implement PCI passthrough and in particular
doesn't define the ioctls used by these wrappers. It might be that the
ppt ioctl interface will require modification to support arm64. Until
that's sorted out one way or another, put this code in a separate file
so that it's easy to conditionally compile.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41003

libvmmapi: Add a subdirectory for amd64-specific code

Move vmmapi_freebsd.c there. It contains x86-specific code used only by
bhyveload(8).

Move vcpu_reset() into vmmapi_machdep.c. It is also x86-specific.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40998