Enable memory ballooning for the Libvirt driver.

Libvirt currently enables the balloon device by default, but it is not used by OpenStack. An initial implementation to take advantage of the device could set a minimum memory guarantee percentage (e.g. 50%) and provide the ability to override this on the flavour extra specifications or instance properties.

As this introduces a difference in how much memory is "available" on a compute node and how much memory is available if all guests burst to their maximum this must also be reflected when scheduling.