vmd(8): only add vionet tap read events after a notify event.

vmd(8): only add vionet tap read events after a notify event.

My recent refactor introduced a bug where the vionet device will
add the tap(4) read event on an unpause regardless of driver and
device state, causing the cpu to peg as the read event fires
indefinitely because the guest's virtqueue isn't ready to receive
the data.

Add in a global flag that tracks if the rx-side is enabled or not
to coordinate adding the tap read event.

ok mlarkin@