devices/console: Fix a bug which would cause libkrun to hang on exit

Make PortInputEmpty and PortInputSigInt `wait_until_readable` implementations
poll the stopfd EventFd to ensure the thread eventually exits.
(The thread is joined in Port::shutdown method causing the hang).

To replicate the hang, the stdin must not be a terminal. For example you can use
a pipe - `echo hello | ./chroot_vm rootfs_fedora /bin/cat` would hang without
this fix.

This seems to be a regresion introduced by the commit:
4076b7: devices/console: implement reset method

Signed-off-by: Matej Hrica <mhrica@redhat.com>

src/devices/src/virtio/console/port_io.rs

@@ -226,8 +226,13 @@ impl PortInput for PortInputSigInt {
         Ok(1)
     }
 
-    fn wait_until_readable(&self, _stopfd: Option<&EventFd>) {
-        let mut poll_fds = [PollFd::new(self.sigint_evt.as_raw_fd(), PollFlags::POLLIN)];
+    fn wait_until_readable(&self, stopfd: Option<&EventFd>) {
+        let mut poll_fds = Vec::new();
+        poll_fds.push(PollFd::new(self.sigint_evt.as_raw_fd(), PollFlags::POLLIN));
+        if let Some(stopfd) = stopfd {
+            poll_fds.push(PollFd::new(stopfd.as_raw_fd(), PollFlags::POLLIN));
+        }
+
         poll(&mut poll_fds, -1).expect("Failed to poll");
     }
 }
@@ -251,7 +256,12 @@ impl PortInput for PortInputEmpty {
         Ok(0)
     }
 
-    fn wait_until_readable(&self, _stopfd: Option<&EventFd>) {
-        std::thread::sleep(std::time::Duration::MAX);
+    fn wait_until_readable(&self, stopfd: Option<&EventFd>) {
+        if let Some(stopfd) = stopfd {
+            let mut poll_fds = [PollFd::new(stopfd.as_raw_fd(), PollFlags::POLLIN)];
+            poll(&mut poll_fds, -1).expect("Failed to poll");
+        } else {
+            std::thread::sleep(std::time::Duration::MAX);
+        }
     }
 }