vspd: Ensure backup loop runs forever.

Fixing a bug where the backup loop was not a loop - it was only running
one time.

cmd/vspd/main.go

@@ -128,6 +128,7 @@ func run() int {
 	// Periodically write a database backup file.
 	shutdownWg.Add(1)
 	go func() {
+		for {
 			select {
 			case <-ctx.Done():
 				shutdownWg.Done()
@@ -138,6 +139,7 @@ func run() int {
 					log.Errorf("Failed to write database backup: %v", err)
 				}
 			}
+		}
 	}()
 
 	// Wait for shutdown tasks to complete before running deferred tasks and