x86/sysctl: Don't return cpu policy data for compiled-out support

x86/sysctl: Don't return cpu policy data for compiled-out support

Policy objects aren't tiny, and the derivation logic isn't trivial.  We are
about to increase the number of policy objects, so will have the opportunity
to drop logic and storage space based on CONFIG_{PV,HVM}.

Start by causing XEN_SYSCTL_get_cpu_policy to fail with -EOPNOTSUPP when
requesting data for a compiled-out subsystem.  Update xen-cpuid to cope and
continue to further system policies, seeing as the indicies are interleaved.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>