Add a driver method to toggle instance booting

Add a driver method to toggle the ability to boot an instance.

The only implementation provided is for the xenapi virt driver.  Other
drivers will log a warning and treat the method as a no-op.

Implements: blueprint cleanup-running-deleted-shutdown

Change-Id: Ie448c681d653b126cb12d3a48d79ab4febde8828
Merge "Add a driver method to toggle instance booting"