KVM: x86: Route pending NMIs from userspace through process_nmi()

Use the asynchronous NMI queue to handle pending NMIs coming in from
userspace during KVM_SET_VCPU_EVENTS so that all of KVM's logic for
handling multiple NMIs goes through process_nmi().  This will simplify
supporting SVM's upcoming "virtual NMI" functionality, which will need
changes KVM manages pending NMIs.
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/x86.c

@@ -5213,9 +5213,9 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
 
 	vcpu->arch.nmi_injected = events->nmi.injected;
 	if (events->flags & KVM_VCPUEVENT_VALID_NMI_PENDING) {
-		vcpu->arch.nmi_pending = events->nmi.pending;
-		if (vcpu->arch.nmi_pending)
-			kvm_make_request(KVM_REQ_NMI, vcpu);
+		vcpu->arch.nmi_pending = 0;
+		atomic_set(&vcpu->arch.nmi_queued, events->nmi.pending);
+		kvm_make_request(KVM_REQ_NMI, vcpu);
 	}
 	static_call(kvm_x86_set_nmi_mask)(vcpu, events->nmi.masked);