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/pmem.rs

@@ -318,7 +318,7 @@ pub struct Pmem {
     config: VirtioPmemConfig,
     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>,
 }
 
@@ -344,7 +344,7 @@ impl Pmem {
             config,
             queue_evts: None,
             interrupt_cb: None,
-            epoll_thread: None,
+            epoll_threads: None,
             paused: Arc::new(AtomicBool::new(false)),
         })
     }
@@ -494,7 +494,7 @@ impl VirtioDevice for Pmem {
                     ActivateError::BadActivate
                 })?;
 
-            self.epoll_thread = Some(epoll_threads);
+            self.epoll_threads = Some(epoll_threads);
 
             return Ok(());
         }