Add support for live_snapshot in compute

This adds live_snapshot support to the compute api, the compute
manager, and the driver interface. The api extension and an
implementation for libvirt will come in follow-on-patches.

Part of blueprint live-snapshot-vms

Change-Id: I0be54d1bbe6057dbaf497b0b71cf68658c0b1832
WIP

Change-Id: Iab6836a4307770a9b1391511ea7488c08a8f430e