Revert "libvirt: reworks configdrive creation"

This reverts commit 228d0221763b12f11ecbacde4db38b1151f96e31.

This commit introduced bug 1369627 (a security vulnerability), as
well as misunderstanding how our image caching works for libvirt.
I think we can achieve the same thing in a much simpler manner.

Change-Id: I7ede9b0f77710869197d9b48c6dd6bbc3087738c
Merge "Revert "libvirt: reworks configdrive creation""