This patch introduces a VirtAPI class which will house
callbacks provided by the manager to the virt drivers, allowing
things such as direct database accesses to be pulled out of
the virt drivers and delegated to another service.
As a first step, this introduces an instance_update() method
and makes all the virt drivers use it instead of direct calls
to db.instance_update.*().
Change-Id: I2e40831f5cfb20a03b304097d84d592aab035ef1