Allow 'tcg' as accelerator in builder-qemu

* 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.
Merge branch 'master_add_support_for_tcg_accel_in_qemu_builder_' of github.com:mattthias/packer into mattthias-master_add_support_for_tcg_accel_in_qemu_builder_

Conflicts:
	builder/qemu/builder.go
	website/source/docs/builders/qemu.html.markdown
Merge branch 'mattthias-master_add_support_for_tcg_accel_in_qemu_builder_'
Update CHANGELOG