Configures the needed module to enable nested virtualization

According to the [kernel source](https://github.com/torvalds/linux/blob/master/Documentation/virtual/kvm/nested-vmx.txt#L48-L49):

> The nested VMX feature is disabled by default. It can be enabled by giving
> the "nested=1" option to the kvm-intel module.

So it looks like this code is correct.