Make create_vm work on all OS versions in use

Makes it work with the same cloudinit code on Ubuntu 20.04 by mounting
it from a custom ISO as CD-ROM

site-cookbooks/kosmos_kvm/templates/create_vm.erb

@@ -64,6 +64,7 @@ EOS
   popd
 fi
 
+<% if @modern_virt_install -%>
 virt-install \
   --name "$VMNAME" \
   --ram "$RAM" \
@@ -83,3 +84,30 @@ virt-install \
   --autostart \
   --import \
   --cloud-init root-password-generate=off,disable=on,meta-data=$CIDATA_PATH/meta-data,user-data=$CIDATA_PATH/user-data
+<% else -%>
+# Legacy path for hosts running virt-install < 4.0.0 (e.g. Ubuntu 20.04,
+# virt-install 2.2.x). These versions predate the --osinfo/--cloud-init
+# options, so we build a NoCloud seed ISO from the same user-data and
+# meta-data files and attach it as a cdrom. The guest receives the
+# exact same cloud-init configuration as on modern hosts.
+cloud-localds "$CIDATA_PATH/seed.iso" "$CIDATA_PATH/user-data" "$CIDATA_PATH/meta-data"
+
+virt-install \
+  --name "$VMNAME" \
+  --ram "$RAM" \
+  --vcpus "$CPUS" \
+  --cpu host \
+  --arch x86_64 \
+  --hvm \
+  --virt-type kvm \
+  --disk "$IMAGE_PATH" \
+  --disk "$CIDATA_PATH/seed.iso,device=cdrom" \
+  --boot hd \
+  --network=bridge=virbr0,model=virtio \
+  --graphics none \
+  --serial pty \
+  --console pty \
+  --channel unix,mode=bind,path=/var/lib/libvirt/qemu/$VMNAME.guest_agent.0,target_type=virtio,name=org.qemu.guest_agent.0 \
+  --autostart \
+  --import
+<% end -%>