Allow 'tcg' as accelerator in builder-qemu

Hi everyone!

I tried to run packer inside a virtual machine (kvm) and run into the "accelerator" parameter which only can be kvm or xen. This pull-request adds 'tcg' as allowed accelerator. The virtual machine will run slower as with kvm or xen as it does not use the virtualization extension but it will run inside a vm (and also on Windows?). 

* Using 'tcg' as accelerator for qemu-system allows packer to run in a
  virtual machine as no kvm or xen support is needed.

* Also document the default behavior if no accelerator is given in the
  documentation.