lguest: don't allow KVM-detection cpuid.

Host might be running under KVM, but we shouldn't allow Guest to think it
can use KVM hypercalls (it can't, and it will embarrass itself if it tries).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

arch/x86/lguest/boot.c

@@ -70,6 +70,7 @@
 #include <asm/i387.h>
 #include <asm/stackprotector.h>
 #include <asm/reboot.h>		/* for struct machine_ops */
+#include <asm/kvm_para.h>
 
 /*G:010
  * Welcome to the Guest!
@@ -455,6 +456,15 @@ static void lguest_cpuid(unsigned int *ax, unsigned int *bx,
 		*ax &= 0xFFFFF0FF;
 		*ax |= 0x00000500;
 		break;
+
+	/*
+	 * This is used to detect if we're running under KVM.  We might be,
+	 * but that's a Host matter, not us.  So say we're not.
+	 */
+	case KVM_CPUID_SIGNATURE:
+		*bx = *cx = *dx = 0;
+		break;
+
 	/*
 	 * 0x80000000 returns the highest Extended Function, so we futureproof
 	 * like we do above by limiting it to known fields.