Allow to use multiple cpus

There is no way to use more than one cpu.
The `ioapi` is off by default and it forces `virtualbox` to use only _single_ cpu (even CORES=4). 

```
Enabling the I/O APIC is required for 64-bit guest operating systems, especially Windows Vista; 
```
```
it is also required if you want to use more than one virtual CPU in a virtual machine.
```
[Virtualbox doc](http://www.virtualbox.org/manual/ch03.html)