Clean up API headers shared with user space

[raphaelning]

HAXM API should be clearly defined in a small set of header files that can be shared with user space (e.g. QEMU). Currently, however, external API definition is not cleanly separated from code specific to HAXM kernel module, and has fallen out of sync with QEMU. See this discussion for details.

Therefore, we should clean up the API headers and draw a line between external and internal code. Ideally, QEMU should be able to include the resulting shared headers as-is.