qemu: Use the supplied context.Context for launching

This will kill the process when the context is cancelled. As using a nil
context is not permitted it is necessary to substitute with a real
context if it is not initialised in the Config struct.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

qemu/qemu.go

@@ -1235,7 +1235,7 @@ type Config struct {
 	// Path is the qemu binary path.
 	Path string
 
-	// Ctx is not used at the moment.
+	// Ctx is the context used when launching qemu.
 	Ctx context.Context
 
 	// Name is the qemu guest name
@@ -1632,7 +1632,12 @@ func LaunchQemu(config Config, logger QMPLog) (string, error) {
 		return "", err
 	}
 
-	return LaunchCustomQemu(config.Ctx, config.Path, config.qemuParams,
+	ctx := config.Ctx
+	if ctx == nil {
+		ctx = context.Background()
+	}
+
+	return LaunchCustomQemu(ctx, config.Path, config.qemuParams,
 		config.fds, nil, logger)
 }
 
@@ -1640,10 +1645,6 @@ func LaunchQemu(config Config, logger QMPLog) (string, error) {
 //
 // The path parameter is used to pass the qemu executable path.
 //
-// The ctx parameter is not currently used but has been added so that the
-// signature of this function will not need to change when launch cancellation
-// is implemented.
-//
 // params is a slice of options to pass to qemu-system-x86_64 and fds is a
 // list of open file descriptors that are to be passed to the spawned qemu
 // process.  The attrs parameter can be used to control aspects of the
@@ -1668,7 +1669,7 @@ func LaunchCustomQemu(ctx context.Context, path string, params []string, fds []*
 	}
 
 	/* #nosec */
-	cmd := exec.Command(path, params...)
+	cmd := exec.CommandContext(ctx, path, params...)
 	if len(fds) > 0 {
 		logger.Infof("Adding extra file %v", fds)
 		cmd.ExtraFiles = fds