KVM: s390: rename __set_cpuflag() to kvm_s390_set_cpuflags()

No need to make this function special. Move it to a header right away.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180123170531.13687-2-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>