KVM: nVMX: Do not report error code when synthesizing VM-Exit from Real Mode

Don't report an error code to L1 when synthesizing a nested VM-Exit and
L2 is in Real Mode.  Per Intel's SDM, regarding the error code valid bit:

  This bit is always 0 if the VM exit occurred while the logical processor
  was in real-address mode (CR0.PE=0).

The bug was introduced by a recent fix for AMD's Paged Real Mode, which
moved the error code suppression from the common "queue exception" path
to the "inject exception" path, but missed VMX's "synthesize VM-Exit"
path.

Fixes: b97f0745 ("KVM: x86: determine if an exception has an error code only when injecting it.")
Cc: stable@vger.kernel.org
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230322143300.2209476-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -3868,7 +3868,12 @@ static void nested_vmx_inject_exception_vmexit(struct kvm_vcpu *vcpu)
 		exit_qual = 0;
 	}
 
-	if (ex->has_error_code) {
+	/*
+	 * Unlike AMD's Paged Real Mode, which reports an error code on #PF
+	 * VM-Exits even if the CPU is in Real Mode, Intel VMX never sets the
+	 * "has error code" flags on VM-Exit if the CPU is in Real Mode.
+	 */
+	if (ex->has_error_code && is_protmode(vcpu)) {
 		/*
 		 * Intel CPUs do not generate error codes with bits 31:16 set,
 		 * and more importantly VMX disallows setting bits 31:16 in the