reboot from vkernel now works in HEAD

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>]

    I was getting annoyed at having to hit a few keys, wait a bit, and hit
    a few more keys to restart my virtual kernel environment, so I made
    'reboot' work inside the virtual kernel.  OMG this is so cool.  I don't
    even need to leave the vkernel's console open any more, it feels like
    just another machine in the machine room and it only takes 18-28 seconds
    to 'reboot'... just enough time to get out of my chair and stretch!

						    -Matt