On said example:

Libvirt 2.5:

Breakpoint 1, 0x00003fffb7c77ba8 in virDomainDiskDefForeachPath (disk=0x200ab490, ignoreOpenFailure=true, iter=0x20011dc0 <add_file_path>,
opaque=0x3fffffffef70) at ../../../src/conf/domain_conf.c:24851
$1 = (virStorageSourcePtr) 0x200ab630
(gdb) p disk->src->path
$2 = 0x200a9ff0 "/var/lib/uvtool/libvirt/images/kvmguest-artful-normal-a2.qcow"
(gdb) p disk->src->backingStore
$3 = (virStorageSourcePtr) 0x200ab720
(gdb) p disk->src->backingStore->path
$4 = 0x200a9e30 "/var/lib/uvtool/libvirt/images/x-uvt-b64-Y29tLnVidW50dS5jbG91ZC5kYWlseTpzZXJ2ZXI6MTcuMTA6cHBjNjRlbCAyMDE3MDcxNg=="

Libvirt 3.5:
Breakpoint 1, 0x00003fffb7c6a3a8 in virDomainDiskDefForeachPath (disk=0x2008cb50, ignoreOpenFailure=true, iter=0x2000f5e0 <add_file_path>,
opaque=0x3fffffffef70) at ../../../src/conf/domain_conf.c:25984
25984 ../../../src/conf/domain_conf.c: No such file or directory.
(gdb) p disk->src->path
$2 = 0x20085800 "/var/lib/uvtool/libvirt/images/kvmguest-artful-normal-a2.qcow"
(gdb) p disk->src->backingStore
$3 = (virStorageSourcePtr) 0x0

So it is the parsing of the XML into objects I have to track down.

Maybe it is even some Ubuntu Delta that no more correctly matches.

Will run on build from upstream master as well before next report.