Revert "libvirt: support live migrate of instances with conf drives"

This reverts commit ecce888c469c62374a3cc43e3cede11d8aa1e799.

Using the image cache is the wrong way to move an image into the same
storage as other instance disks. We should just call into the relevant
backend store directly.

Change-Id: Id867b89d959c26d067cb06b52a496e2b98573289
Merge "Revert "libvirt: support live migrate of instances with conf drives""