Comment 6 for bug 1823676

Simplified, but complete test steps (working on a fresh Bionic x86 host):
$ sudo apt install uvtool-libvirt
$ time uvt-simplestreams-libvirt --verbose sync --source http://cloud-images.ubuntu.com/daily arch=amd64 label=daily release=bionic
$ ssh-keygen
$ uvt-kvm create --password ubuntu b-testattach arch=amd64 release=bionic label=daily
$ virsh dumpxml b-testattach | grep controller.*scsi
# will show no output - ok
$ echo "<controller type='scsi' model='virtio-scsi' />" > controller_scsi.xml
# check persistent and live config
$ virsh attach-device b-testattach controller_scsi.xml --persistent
error: Failed to attach device from controller_scsi.xml
error: internal error: Cannot parse controller index -1
$ virsh attach-device b-testattach controller_scsi.xml --live --config
error: Failed to attach device from controller_scsi.xml
error: internal error: Cannot parse controller index -1

With the fix those attaches should work just fine.