platform: allow CLC templating for dynamic IP address insertion

The tests that require coreos-metadata and CLC templating could not
run because for QEMU only a static replacement was done before the
CLC configuration was rendered.
Add support for the template variables by treating QEMU as a custom
platform for the transpiler, making use of a coreos-metadata service
that has the IP addresses hardcoded.

platform/local/flight.go

@@ -50,7 +50,7 @@ func NewLocalFlight(opts *platform.Options, platformName platform.Name) (*LocalF
 	}
 
 	nsdialer := network.NewNsDialer(nshandle)
-	bf, err := platform.NewBaseFlightWithDialer(opts, platformName, "", nsdialer)
+	bf, err := platform.NewBaseFlightWithDialer(opts, platformName, "custom", nsdialer)
 	if err != nil {
 		nshandle.Close()
 		return nil, fmt.Errorf("creating new base flight failed: %v", err)

platform/machine/qemu/cluster.go

@@ -59,15 +59,27 @@ func (qc *Cluster) NewMachineWithOptions(userdata *conf.UserData, options platfo
 	ip := strings.Split(netif.DHCPv4[0].String(), "/")[0]
 
 	conf, err := qc.RenderUserData(userdata, map[string]string{
-		"$public_ipv4":  ip,
-		"$private_ipv4": ip,
+		"$public_ipv4":  "${COREOS_CUSTOM_PUBLIC_IPV4}",
+		"$private_ipv4": "${COREOS_CUSTOM_PRIVATE_IPV4}",
 	})
 	if err != nil {
 		qc.mu.Unlock()
 		return nil, err
 	}
 	qc.mu.Unlock()
 
+	conf.AddSystemdUnit("coreos-metadata.service", `[Unit]
+Description=QEMU metadata agent
+After=nss-lookup.target
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+Environment=OUTPUT=/run/metadata/coreos
+ExecStart=/usr/bin/mkdir --parent /run/metadata
+ExecStart=/usr/bin/bash -c 'echo "COREOS_CUSTOM_PRIVATE_IPV4=` + ip + `\nCOREOS_CUSTOM_PUBLIC_IPV4=` + ip + `\n" > ${OUTPUT}'`, false)
+
 	var confPath string
 	if conf.IsIgnition() {
 		confPath = filepath.Join(dir, "ignition.json")