lguest: restrict CPUID to avoid perf counter wrmsr

Avoid the following:
[    0.012093] WARNING: at arch/x86/kernel/apic/apic.c:249 native_apic_write_dummy+0x2f/0x40()

Rather than chase each new cpuid-detected feature, just lie about the highest
valid CPUID so this code is never run.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

arch/x86/lguest/boot.c

@@ -379,6 +379,11 @@ static void lguest_cpuid(unsigned int *ax, unsigned int *bx,
 
 	native_cpuid(ax, bx, cx, dx);
 	switch (function) {
+	case 0: /* ID and highest CPUID.  Futureproof a little by sticking to
+		 * older ones. */
+		if (*ax > 5)
+			*ax = 5;
+		break;
 	case 1:	/* Basic feature request. */
 		/* We only allow kernel to see SSE3, CMPXCHG16B and SSSE3 */
 		*cx &= 0x00002201;