Revert "libvirt: Uses correct imagebackend for configdrive"

This reverts commit 0b01e846d40f3b343da9ebe1dae89cca8bc2ac66.

We  should not be adding instance specific disks with configuration
information like passwords into a shared cache. I think this is the
wrong way of moving the config drive disk image into an arbitrary
backend store.

Change-Id: I1376ef5f3fcbd66d916ff9442efea0f756dd42da
Merge "Revert "libvirt: Uses correct imagebackend for configdrive""