KVM: x86: fix pvclock guest stopped flag reporting

[Marcelo Tosatti <mtosatti@redhat.com>]

kvm_guest_time_update unconditionally clears hv_clock.flags field, 
so the notification never reaches the guest.

Fix it by allowing PVCLOCK_GUEST_STOPPED to passthrough.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3a53bcc..e8ce10f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1217,7 +1217,9 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
 	vcpu->last_kernel_ns = kernel_ns;
 	vcpu->last_guest_tsc = tsc_timestamp;
-	vcpu->hv_clock.flags = 0;
+	/* only support PVCLOCK_GUEST_STOPPED flag ATM */
+	if (vcpu->hv_clock.flags != PVCLOCK_GUEST_STOPPED)
+		vcpu->hv_clock.flags = 0;
 
 	/*
 	 * The interface expects us to write an even number signaling that the