KVM: x86: fix incorrect comparison in trace event

The "u" field in the event has three states, -1/0/1.  Using u8 however means that
comparison with -1 will always fail, so change to signed char.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmutrace.h

@@ -339,7 +339,7 @@ TRACE_EVENT(
 		/* These depend on page entry type, so compute them now.  */
 		__field(bool, r)
 		__field(bool, x)
-		__field(u8, u)
+		__field(signed char, u)
 	),
 
 	TP_fast_assign(