Add one configuration item for cache-using in libvirt/hypervisor

[wingwj]

We met one scenario which needs to close the cache on linux hypervisor.

But some codes written in libvirt/driver.py (including 'suspend'/'snapshot' actions) are hard-coded.

For example:
-----------
    def suspend(self, instance):
        """Suspend the specified instance."""
        dom = self._lookup_by_name(instance['name'])
        self._detach_pci_devices(dom,
            pci_manager.get_instance_pci_devs(instance))
        dom.managedSave(0)

So, we need to add one configuration item for it in nova.conf, and let operator can choose it.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/77782

[Joe Gordon (jogo)]

Is this bug still valid, the patch has been abandoned.