virtio-ccw: Wire up ioeventfd.

On hosts that support ioeventfd, make use of it for host-to-guest
notifications via diagnose 500.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
virtio-ccw: Wire up guest and host notifies.

Guest and host notifiers are needed by vhost. We use ioeventfds for
the guest notifiers, but need to fall back on qemu injecting interrupts
for the host notifiers.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>