virtio-devices, net_util: Migrate virtio-net to EpollHelper

EpollHelper allows the removal of much duplicated loop handling code and
instead the device specific even handling is delegated via an
implementation of EpollHelperHandler.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vhost_user_net/src/lib.rs

@@ -110,6 +110,7 @@ impl VhostUserNetThread {
                 rx_tap_listening: false,
                 epoll_fd: None,
                 counters: NetCounters::default(),
+                tap_event_id: 2,
             },
         })
     }