lifecycle: align "KILL_EVENT received" messages across code-base

To align the logging messages with the rest of the code, this
message should be aligned with another similar occurrence in
epoll_helper.rs

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

vmm/src/serial_manager.rs

@@ -300,7 +300,7 @@ impl SerialManager {
                                     }
                                 }
                                 EpollDispatch::Kill => {
-                                    info!("KILL event received, stopping epoll loop");
+                                    info!("KILL_EVENT received, stopping epoll loop");
                                     return Ok(());
                                 }
                             }