KVM: x86: Return UNHANDLABLE on unsupported SYSENTER

Now that KVM injects #UD on "unhandlable" error, it makes better sense to
return such error on sysenter instead of directly injecting #UD to the guest.
This allows to track more easily the unhandlable cases the emulator does not
support.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -2358,11 +2358,9 @@ static int em_sysenter(struct x86_emulate_ctxt *ctxt)
 	    && !vendor_intel(ctxt))
 		return emulate_ud(ctxt);
 
-	/* XXX sysenter/sysexit have not been tested in 64bit mode.
-	* Therefore, we inject an #UD.
-	*/
+	/* sysenter/sysexit have not been tested in 64bit mode. */
 	if (ctxt->mode == X86EMUL_MODE_PROT64)
-		return emulate_ud(ctxt);
+		return X86EMUL_UNHANDLEABLE;
 
 	setup_syscalls_segments(ctxt, &cs, &ss);