Pause between boot_commands when debugging a QEMU build

When debugging a build (or maintaining an existing packer template), teach `packer build -debug` how to step through individual `boot_command`s in order to triage the packer template.

builder/qemu/builder.go

@@ -379,15 +379,18 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 	state := new(multistep.BasicStateBag)
 	state.Put("cache", cache)
 	state.Put("config", &b.config)
+	state.Put("debug", b.config.PackerDebug)
 	state.Put("driver", driver)
 	state.Put("hook", hook)
 	state.Put("ui", ui)
 
 	// Run
 	if b.config.PackerDebug {
+		pauseFn := common.MultistepDebugFn(ui)
+		state.Put("pauseFn", pauseFn)
 		b.runner = &multistep.DebugRunner{
 			Steps:   steps,
-			PauseFn: common.MultistepDebugFn(ui),
+			PauseFn: pauseFn,
 		}
 	} else {
 		b.runner = &multistep.BasicRunner{Steps: steps}

builder/qemu/step_type_boot_command.go

@@ -38,10 +38,16 @@ type stepTypeBootCommand struct{}
 
 func (s *stepTypeBootCommand) Run(state multistep.StateBag) multistep.StepAction {
 	config := state.Get("config").(*Config)
+	debug := state.Get("debug").(bool)
 	httpPort := state.Get("http_port").(uint)
 	ui := state.Get("ui").(packer.Ui)
 	vncPort := state.Get("vnc_port").(uint)
 
+	var pauseFn multistep.DebugPauseFn
+	if debug {
+		pauseFn = state.Get("pauseFn").(multistep.DebugPauseFn)
+	}
+
 	// Connect to VNC
 	ui.Say("Connecting to VM via VNC")
 	nc, err := net.Dial("tcp", fmt.Sprintf("127.0.0.1:%d", vncPort))
@@ -72,7 +78,7 @@ func (s *stepTypeBootCommand) Run(state multistep.StateBag) multistep.StepAction
 	}
 
 	ui.Say("Typing the boot command over VNC...")
-	for _, command := range config.BootCommand {
+	for i, command := range config.BootCommand {
 		command, err := interpolate.Render(command, &ctx)
 		if err != nil {
 			err := fmt.Errorf("Error preparing boot command: %s", err)
@@ -87,6 +93,10 @@ func (s *stepTypeBootCommand) Run(state multistep.StateBag) multistep.StepAction
 			return multistep.ActionHalt
 		}
 
+		if pauseFn != nil {
+			pauseFn(multistep.DebugLocationAfterRun, fmt.Sprintf("boot_command[%d]: %s", i, command), state)
+		}
+
 		vncSendString(c, command)
 	}