Make libvirt use device aliases for detach

Currently we identify devices in Libvirt guest XML by a variety of methods, which differs based on the device type (at least). Libvirt now provides a device alias mechanism by which we can tie virtual guest devices to an identifier we can use to look them up in a stable and generic way. Nova should move to using that, which will increase consistency, decrease some complexity, and also work around some issues with our current strategy.