KVM: s390: implements the kvm_arch_vcpu_in_kernel()

This implements kvm_arch_vcpu_in_kernel() for s390.  DIAG is a privileged
operation, so it cannot be called from problem state (user mode).

Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>