wip: rbgrun qemu wrapper for testing

Blocking: until cargo supports [target.*] runner = ".."

Development may continue as rgbrun may be called manually. This currently only respects the number of cores.

• Multicore -smp cores=$CONFIG_MAX_NUM_NODES, if >1
• Kernel target architecture (currently assuming x86_64)
• <insert more configuration-specific emulator options>
• Autodetect TAP from stdout and run the TAP harness (waiting on meta#35)