Add tap(4) read events after rx virtqueue notification.

Add tap(4) read events after rx virtqueue notification.

The recent vmd(8) vionet refactor caused cpu spinning when the
device would be activated by a driver but before virtqueues were
provided. (e.g. booting a ramdisk kernel and not running `ifconfig
vio0 up`)

Defer adding the tap and packet injection read events until after
the first RX queue notification from the driver.

ok mlarkin@