virtio-ccw: Check indicators location.

virtio-ccw: Check indicators location.

If a guest neglected to register (secondary) indicators but still runs
with notifications enabled, we might end up writing to guest zero;
avoid this by checking for valid indicators and only writing to the
guest and generating an interrupt if indicators have been setup.

Cc: qemu-stable@nongnu.org
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
(cherry picked from commit 7c4869761d7f2e0a3f806a5359eea5d2473ec5d5)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>