vmm: do not start signal thread to resize console if no need

Now cloud hypervisor will start signal thread to catch
SIGWINCH signal, cloud hypervisor then will resize the
guest console via vconsole.

This patch skip starting signal thread when there is no
need to resize guest console, such as console is not
configured.

Signed-off-by: Yong He <alexyonghe@tencent.com>

vmm/src/vm.rs

@@ -1943,6 +1943,11 @@ impl Vm {
     fn setup_signal_handler(&mut self) -> Result<()> {
         let console = self.device_manager.lock().unwrap().console().clone();
         let signals = Signals::new(Vm::HANDLED_SIGNALS);
+
+        if !console.need_resize() {
+            return Ok(());
+        }
+
         match signals {
             Ok(signals) => {
                 self.signals = Some(signals.handle());