x86/spec-ctrl: Hide RDRAND by default on IvyBridge client

x86/spec-ctrl: Hide RDRAND by default on IvyBridge client

To combat the absence of mitigating microcode, arrange to hide RDRAND by
default on IvyBridge client hardware.

Adjust the default feature derivation to hide RDRAND on IvyBridge client
parts, unless `cpuid=rdrand` is explicitly provided.

Adjust the restore path in xc_cpuid_apply_policy() to not hide RDRAND from VMs
which migrated from pre-4.14.

In all cases, individual guests can continue using RDRAND if explicitly
enabled in their config files.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Paul Durrant <paul@xen.org>