s390-sclp: SCLP Event integration

Add an sclp event for "cpu was hot plugged".  This allows Qemu to deliver an
SCLP interrupt to the guest stating that the requested cpu hotplug was
completed.

Signed-off-by: Jason J. Herne <jjherne@us.ibm.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>