s390/pci: implement hotplug notifications

When the availability of a pci function has changed by means
outside of our control we receive an availability event.
Implement/improve the handling of these notifications.

Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>