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.