apparmor: let image label setting loop over backing files

apparmor: let image label setting loop over backing files

When adding a rule for an image file and that image file has a chain
of backing files then we need to add a rule for each of those files.

To get that iterate over the backing file chain the same way as
dac/selinux already do and add a label for each.

Fixes: https://gitlab.com/libvirt/libvirt/-/issues/118
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>