KVM: s390: Invoke kvm_vcpu_init() before allocating sie_page

Now that s390's implementation of kvm_arch_vcpu_init() is empty, move
the call to kvm_vcpu_init() above the allocation of the sie_page.  This
paves the way for moving vcpu allocation and initialization into common
KVM code without any associated functional change.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>