Extend the net backend interface with two functions, namely netbe_rx_disable()
and netbe_rx_enable(), which can be used by the net device emulators to stop
the backend from invoking the receive callback. This is useful for the device emulators,
i.e. on hardware resets or to implement receive backpressure.
To prevent race conditions, the net backends starts with receive operation disabled.
The mevent module has been extended to support the addition of a disabled event.
A follow-up patch will use the new functionalities in the virtio-net device.