We've had several bugs about this over the years, and until we
actually decide to fix it (or not), we should point out the known
limitation that volume-backed root disks are not replaced during
a rebuild.
Like, if you have a volume-backed instance and rebuild with a new
image, the root disk is still the volume with the original image.
Change-Id: I145cab88f782e4b1e630cc432322bc8436413e71
Related-Bug: #1482040