x86: don't expose SYSENTER on unknown CPUs

So far we only ever set up the respective MSRs on Intel CPUs, yet we
hide the feature only on a 32-bit hypervisor. That prevents booting of
PV guests on top of a 64-bit hypervisor making use of the instruction
on unknown CPUs (VIA in this case).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -55,6 +55,7 @@ static void default_init(struct cpuinfo_
 	/* Not much we can do here... */
 	/* Check if at least it has cpuid */
 	BUG_ON(c->cpuid_level == -1);
+	__clear_bit(X86_FEATURE_SEP, c->x86_capability);
 }
 
 static struct cpu_dev default_cpu = {