This is a helper for implementing the worker thread for virtio devices
and in particular handles special behaviour for pause and kill events.
The device specific event handling (for the queues themselves) is
delegated to a method invoked on a new EpollHelperHandler trait. This
method is passed the event as well as the EpollHelper so that it can
operate on the handler in order to manage events itself (required for
virtio-net.)
Signed-off-by: Rob Bradford <robert.bradford@intel.com>
