Add CPU hotplug feature

[hedgehog]

config.vm.customize do |vm|
  vm.cpu_count = 6
  vm.cpu.hotplug = true
end

$ vagrant unplugcpu 2
$ vagrant unplugcpu 3
$ vagrant unplugcpu 4
$ vagrant unplugcpu 5
$ vagrant plugcpu 2
$ vagrant cpus
mximum: 6
active: 3

Note that this makes it less likely that VirtualBox OSE is fully supported, since hotplugging requires Guest Additions be installed, and of OSE it seems the only (Ubuntu) package has a X11 dependency....