KVM: s390: rework code that sets the prefix

There are several places in the kvm module, which set the prefix register.
Since we need to flush the cpu, lets combine this operation into a helper
function. This helper will also explicitely mask out the unused bits.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index 278ee00..c6366cf 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -236,8 +236,7 @@
 		VCPU_EVENT(vcpu, 4, "interrupt: set prefix to %x",
 			   inti->prefix.address);
 		vcpu->stat.deliver_prefix_signal++;
-		vcpu->arch.sie_block->prefix = inti->prefix.address;
-		vcpu->arch.sie_block->ihcpu = 0xffff;
+		kvm_s390_set_prefix(vcpu, inti->prefix.address);
 		break;
 
 	case KVM_S390_RESTART: