lguest: fix crash 'unhandled trap 13 at <native_read_msr_safe>'

Impact: fix lguest boot crash on modern Intel machines

The code in early_init_intel does:

	if (c->x86 > 6 || (c->x86 == 6 && c->x86_model >= 0xd)) {
		u64 misc_enable;

		rdmsrl(MSR_IA32_MISC_ENABLE, misc_enable);

And that rdmsr faults (not allowed from non-0 PL).  We can get around
this by mugging the family ID part of the cpuid.  5 seems like a good
number.

Of course, this is a hack (how very lguest!).  We could just indicate
that we don't support MSRs, or implement lguest_rdmst.
Reported-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Tested-by: Patrick McHardy <kaber@trash.net>

arch/x86/lguest/boot.c

@@ -343,6 +343,11 @@ static void lguest_cpuid(unsigned int *ax, unsigned int *bx,
 		 * flush_tlb_user() for both user and kernel mappings unless
 		 * the Page Global Enable (PGE) feature bit is set. */
 		*dx |= 0x00002000;
+		/* We also lie, and say we're family id 5.  6 or greater
+		 * leads to a rdmsr in early_init_intel which we can't handle.
+		 * Family ID is returned as bits 8-12 in ax. */
+		*ax &= 0xFFFFF0FF;
+		*ax |= 0x00000500;
 		break;
 	case 0x80000000:
 		/* Futureproof this a little: if they ask how much extended