s390x/kvm: Handle bpb feature

We need to handle the bpb control on reset and migration. Normally
stfle.82 is transparent (and the normal guest part works without
hypervisor activity). To prevent any issues we require full
host kernel support for this feature.

Cc: qemu-stable@nongnu.org
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20180118085628.40798-3-borntraeger@de.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
[CH: 'Branch Prediction Blocking' -> 'Branch prediction blocking']
Signed-off-by: Cornelia Huck <cohuck@redhat.com>