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>

vmm/src/memory_manager.rs

@@ -26,7 +26,7 @@ use std::ops::Deref;
 use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
 use std::path::PathBuf;
 use std::result;
-use std::sync::{Arc, Mutex};
+use std::sync::{Arc, Barrier, Mutex};
 use url::Url;
 #[cfg(target_arch = "x86_64")]
 use vm_allocator::GsiApic;
@@ -314,7 +314,7 @@ impl BusDevice for MemoryManager {
         }
     }
 
-    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) {
+    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
         match offset {
             SELECTION_OFFSET => {
                 self.selected_slot = usize::from(data[0]);
@@ -340,7 +340,8 @@ impl BusDevice for MemoryManager {
                     offset
                 );
             }
-        }
+        };
+        None
     }
 }