Controller: emit lifecycle events when the VM gets restarted or deleted (#208)

* Controller: emit lifecycle events when the VM gets restarted or deleted

* vm_{scheduling,run}_time → vm_{scheduling,run}_duration for clarity

* Update VM endpoint: only update VM started time when zero

pkg/resource/v1/v1.go

@@ -57,6 +57,9 @@ type VM struct {
 	// by the worker using "tart fqn" command after it had pulled the image.
 	ImageFQN string `json:"image_fqn,omitempty"`
 
+	ScheduledAt time.Time `json:"scheduled_at,omitempty"`
+	StartedAt   time.Time `json:"started_at,omitempty"`
+
 	Meta
 }