vm-virtio: Modify VirtioInterrupt callback into a trait

Callbacks are not the most Rust idiomatic way of programming. The right
way is to use a Trait to provide multiple implementation of the same
interface.

Additionally, a Trait will allow for multiple functions to be defined
while using callbacks means that a new callback must be introduced for
each new function we want to add.

For these two reasons, the current commit modifies the existing
VirtioInterrupt callback into a Trait of the same name.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/vhost_user/net.rs

@@ -44,7 +44,7 @@ pub struct Net {
     config_space: Vec<u8>,
     queue_sizes: Vec<u16>,
     queue_evts: Option<Vec<EventFd>>,
-    interrupt_cb: Option<Arc<VirtioInterrupt>>,
+    interrupt_cb: Option<Arc<dyn VirtioInterrupt>>,
     epoll_thread: Option<Vec<thread::JoinHandle<result::Result<(), DeviceError>>>>,
     ctrl_queue_epoll_thread: Option<thread::JoinHandle<result::Result<(), CtrlError>>>,
     paused: Arc<AtomicBool>,
@@ -227,7 +227,7 @@ impl VirtioDevice for Net {
     fn activate(
         &mut self,
         mem: Arc<ArcSwap<GuestMemoryMmap>>,
-        interrupt_cb: Arc<VirtioInterrupt>,
+        interrupt_cb: Arc<dyn VirtioInterrupt>,
         mut queues: Vec<Queue>,
         mut queue_evts: Vec<EventFd>,
     ) -> ActivateResult {
@@ -336,7 +336,7 @@ impl VirtioDevice for Net {
         Ok(())
     }
 
-    fn reset(&mut self) -> Option<(Arc<VirtioInterrupt>, Vec<EventFd>)> {
+    fn reset(&mut self) -> Option<(Arc<dyn VirtioInterrupt>, Vec<EventFd>)> {
         // We first must resume the virtio thread if it was paused.
         if self.pause_evt.take().is_some() {
             self.resume().ok()?;