Split vmd into mi/md parts.

Split vmd into mi/md parts.

Makes as much of the core of vmd mi, pushing x86-isms into separate
compilation units. Adds build logic for arm64, but no emulation
yet. (You can build vmd, but it won't have a vmm device to connect
to.)

Some more cleanup probably needed around interrupt controller
abstraction, but that can come as we implement more than the i8259.

ok mlarkin@