fix: Refactor vmm builder code to simplify logic

fix up aarch64 attach_legacy_devices_aarch64 fn

Signed-off-by: tommady <tommady@users.noreply.github.com>

src/vmm/src/builder.rs

@@ -168,7 +168,9 @@ pub mod aarch64 {
         if cmdline_contains_console {
             // Make stdout non-blocking.
             set_stdout_nonblocking();
-            let serial = setup_serial_device(event_manager, std::io::stdin(), std::io::stdout())?;
+            let serial = setup_serial_device(std::io::stdin(), std::io::stdout())?;
+            event_manager.add_subscriber(serial.clone());
+
             vmm.mmio_device_manager
                 .register_mmio_serial(vmm.vm.fd(), &mut vmm.resource_allocator, serial, None)
                 .map_err(VmmError::RegisterMMIODevice)?;