Add the new graceful_halt settings to `config.vm`
GracefulHalt builtin
Use the new GracefulHalt built-in for VirtualBox
Guest `halt` implementations no longer depend on machine state
Better logging in the GracefulHalt middleware
Call built-in supports args for imddleware
Merge branch 'graceful-halt-builtin'

This introduces a new middleware builtin: GracefulHalt. This should be
used to gracefully shut down the machine. It removes any concept of
timeouts and such out of the guests.