KVM: s390: Fixed priority of execution in STSI

Added some missing validity checks for the operands and fixed the
priority of exceptions for some function codes according to the
"Principles of Operation" document.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>