Just as libvirt needs to decide on the device names when booting an
instance (as opposed to them being assigned by the user), the same needs
to be done when attaching additional volumes to a running instance.
This patch exposes it on the ComputeDriver base class and makes sure
that it will be called by the compute manager (and that an appropriate
fallback will happen if the drivers do not care about overriding this),
as a pre-req for doing the work in the libvirt driver.
Partial-bug: 1452224
Related-bug: 1231874
Change-Id: I66693a8a6a632b65f930a58905336397ac6c9f29
b6733dbdf3