vm-virtio: Rename epoll_thread to epoll_threads

Now that we unified epoll_thread to potentially be a vector of threads,
it makes sense to make it a plural field.

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

vm-virtio/src/iommu.rs

@@ -765,7 +765,7 @@ pub struct Iommu {
     ext_mapping: BTreeMap<u32, Arc<dyn ExternalDmaMapping>>,
     queue_evts: Option<Vec<EventFd>>,
     interrupt_cb: Option<Arc<dyn VirtioInterrupt>>,
-    epoll_thread: Option<Vec<thread::JoinHandle<result::Result<(), DeviceError>>>>,
+    epoll_threads: Option<Vec<thread::JoinHandle<result::Result<(), DeviceError>>>>,
     paused: Arc<AtomicBool>,
 }
 
@@ -795,7 +795,7 @@ impl Iommu {
                 ext_mapping: BTreeMap::new(),
                 queue_evts: None,
                 interrupt_cb: None,
-                epoll_thread: None,
+                epoll_threads: None,
                 paused: Arc::new(AtomicBool::new(false)),
             },
             mapping,
@@ -948,7 +948,7 @@ impl VirtioDevice for Iommu {
                 ActivateError::BadActivate
             })?;
 
-        self.epoll_thread = Some(epoll_threads);
+        self.epoll_threads = Some(epoll_threads);
 
         Ok(())
     }