s390/sthyi: reorganize sthyi implementation

As we need to support sthyi instruction on LPAR too, move the common code
to kernel part and kvm related code to intercept.c for better reuse.

Signed-off-by: QingFeng Hao <haoqf@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>