libvmmapi: Move some ioctl wrappers to vmmapi_machdep.c

ioctls relating to segments and various x86-specific interrupt
controllers are easy candidates to move to vmmapi_machdep.c.

In vmmapi.h I'm just ifdefing MD prototypes for now. We could instead
split vmmapi.h into multiple headers, e.g., vmmapi.h and
vmmapi_machdep.h, but it's not obvious to me yet that that's the right
approach.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40999