misc: Make Bus::write() return an Option<Arc<Barrier>>

This can be uses to indicate to the caller that it should wait on the
barrier before returning as there is some asynchronous activity
triggered by the write which requires the KVM exit to block until it's
completed.

This is useful for having vCPU thread wait for the VMM thread to proceed
to activate the virtio devices.

See #1863

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

devices/src/legacy/serial.rs

@@ -7,7 +7,7 @@
 
 use anyhow::anyhow;
 use std::collections::VecDeque;
-use std::sync::Arc;
+use std::sync::{Arc, Barrier};
 use std::{io, result};
 use vm_device::interrupt::InterruptSourceGroup;
 use vm_device::BusDevice;
@@ -275,12 +275,14 @@ impl BusDevice for Serial {
         };
     }
 
-    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) {
+    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
         if data.len() != 1 {
-            return;
+            return None;
         }
 
-        if let Err(_e) = self.handle_write(offset as u8, data[0]) {}
+        self.handle_write(offset as u8, data[0]).ok();
+
+        None
     }
 }