enable libvirt at boot

When using libvirt driver, we need to ensure that libvirt service is
actually enabled at boot.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>

Conflicts:
	spec/classes/nova_compute_libvirt_spec.rb

Change-Id: Ia3438df1d64761760d0f601c71740158bd4f6460