runtime-rs: open stdout and stderr fifos NONBLOCK

This patch adds O_NONBLOCK flag when open stdout and stderr FIFOs
to avoid blocking.

Fixes: #6714
Signed-off-by: Zixuan Tan <tanzixuan.me@gmail.com>

src/runtime-rs/crates/runtimes/virt_container/src/container_manager/io/passfd_io.rs

@@ -98,6 +98,7 @@ impl PassfdIo {
         if let Some(stdout) = &self.stdout {
             let fout = OpenOptions::new()
                 .write(true)
+                .custom_flags(libc::O_NONBLOCK)
                 .open(&stdout)
                 .context("open stdout")?;
 
@@ -113,6 +114,7 @@ impl PassfdIo {
             if let Some(stderr) = &self.stderr {
                 let ferr = OpenOptions::new()
                     .write(true)
+                    .custom_flags(libc::O_NONBLOCK)
                     .open(&stderr)
                     .context("open stderr")?;