vmm/virtualbox: Enable exception tracebacks if VERBOSE is set
vmm/virtualbox: Enable exception tracebacks if VERBOSE is set

- Enable vmm.DEBUG if the VERBOSE input variable is set; disable it
otherwise

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr