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/priv.c b/arch/s390/kvm/priv.c
index d026389..9c83b8a 100644
--- a/arch/s390/kvm/priv.c
+++ b/arch/s390/kvm/priv.c
@@ -56,8 +56,7 @@
 		goto out;
 	}
 
-	vcpu->arch.sie_block->prefix = address;
-	vcpu->arch.sie_block->ihcpu = 0xffff;
+	kvm_s390_set_prefix(vcpu, address);
 
 	VCPU_EVENT(vcpu, 5, "setting prefix to %x", address);
 out: