virtio-ccw: implement ->device_plugged

Let's move operations that are only valid after the backend has been
realized to a ->device_plugged callback, just as virtio-pci does.
Also reorder setting up the host feature bits to the sequence used
by virtio-pci.

While we're at it, also add a ->device_unplugged callback to stop
ioeventfd, just to be on the safe side.

Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Message-Id: <1429627016-30656-3-git-send-email-cornelia.huck@de.ibm.com>