Commit aa9e0057b1 made the libvirt backend
use <shareable/> for the disk of the appliance, since at that time all
the instances were using the disk directly.
OTOH, commit 3ad44c8660 switched to
overlays for read-only disks, including the appliance, so effectively
protecting the appliance.
Because of this, the libvirt backend does not need <shareable/> anymore.
Thanks to: Daniel Berrange, Richard W.M. Jones, Peter Krempa.