vm-virtio: Implement the Pausable trait for all virtio devices

Due to the amount of code currently duplicated across virtio devices,
the stats for this commit is on the large side but it's mostly more
duplicated code, unfortunately.

Migratable and Snapshotable placeholder implementations are provided as
well, making all virtio devices Migratable.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

vm-virtio/src/device.rs

@@ -101,3 +101,45 @@ pub trait VirtioDevice: Send {
 pub trait DmaRemapping: Send + Sync {
     fn translate(&self, id: u32, addr: u64) -> std::result::Result<u64, std::io::Error>;
 }
+
+#[macro_export]
+macro_rules! virtio_pausable_inner {
+    () => {
+        fn pause(&mut self) -> result::Result<(), MigratableError> {
+            debug!(
+                "Pausing virtio-{}",
+                VirtioDeviceType::from(self.device_type())
+            );
+            self.paused.store(true, Ordering::SeqCst);
+            if let Some(pause_evt) = &self.pause_evt {
+                pause_evt
+                    .write(1)
+                    .map_err(|e| MigratableError::Pause(e.into()))?;
+            }
+
+            Ok(())
+        }
+
+        fn resume(&mut self) -> result::Result<(), MigratableError> {
+            debug!(
+                "Resuming virtio-{}",
+                VirtioDeviceType::from(self.device_type())
+            );
+            self.paused.store(false, Ordering::SeqCst);
+            if let Some(epoll_thread) = &self.epoll_thread {
+                epoll_thread.thread().unpark();
+            }
+
+            Ok(())
+        }
+    }
+}
+
+#[macro_export]
+macro_rules! virtio_pausable {
+    ($name:ident) => {
+        impl Pausable for $name {
+            virtio_pausable_inner!();
+        }
+    };
+}