vmm: clean up migration threads on failure

Migration threads may be left orphaned and keep the socket bound after
a migration has failed. Prevent this by signaling termination via the
`kill_evt` in `ReceiveAdditionalConnections`'s `Drop` impl.

On-behalf-of: SAP julian.schindel@sap.com
Signed-off-by: Julian Schindel <julian.schindel@cyberus-technology.de>

vmm/src/migration/transport.rs

@@ -589,6 +589,9 @@ impl ReceiveAdditionalConnections {
 
 impl Drop for ReceiveAdditionalConnections {
     fn drop(&mut self) {
+        if let Err(error) = self.kill_evt.write(1) {
+            warn!("Failed to write to kill eventfd: {error}");
+        }
         if self.accept_thread.is_some() {
             warn!(
                 "ReceiveAdditionalConnections was not cleaned up! Either cleanup() was never called (programming error) or it failed before completing."