kvm: x86: Pass through AMD_STIBP_ALWAYS_ON in GET_SUPPORTED_CPUID

This bit is purely advisory. Passing it through to the guest indicates
that the virtual processor, like the physical processor, prefers that
STIBP is only set once during boot and not changed.

Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>