KVM: VMX: Relax check on unusable segment

Some userspace (e.g. QEMU 1.1) munge the d and g bits of segment
descriptors, causing us not to recognize them as unusable segments
with emulate_invalid_guest_state=1.  Relax the check by testing for
segment not present (a non-present segment cannot be usable).
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/vmx.c

@@ -3198,7 +3198,7 @@ static u32 vmx_segment_access_rights(struct kvm_segment *var)
 {
 	u32 ar;
 
-	if (var->unusable)
+	if (var->unusable || !var->present)
 		ar = 1 << 16;
 	else {
 		ar = var->type & 15;
@@ -3210,8 +3210,6 @@ static u32 vmx_segment_access_rights(struct kvm_segment *var)
 		ar |= (var->db & 1) << 14;
 		ar |= (var->g & 1) << 15;
 	}
-	if (ar == 0) /* a 0 value means unusable */
-		ar = AR_UNUSABLE_MASK;
 
 	return ar;
 }