s390: sclp signal quiesce support

s390: sclp signal quiesce support

This implements the sclp signal quiesce event via the SCLP Event
Facility.
This allows to gracefully shutdown a guest by using system_powerdown
notifiers. It creates a service interrupt that will trigger a
Read Event Data command from the guest. This code will then add an
event that is interpreted by linux guests as ctrl-alt-del.

Signed-off-by: Heinz Graalfs <[email protected]>
Signed-off-by: Christian Borntraeger <[email protected]>
Signed-off-by: Jens Freimann <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>