libx86: Introduce a helper to serialise cpuid_policy objects

libx86: Introduce a helper to serialise cpuid_policy objects

The serialised form is made up of the leaf, subleaf and data tuple.  As this
is the architectural form, it is expected not to change going forwards.

The serialisation of the Xen/Viridian leaves isn't fully implemented yet.  It
is just enough to be bug-compatible with the current DOMCTL_set_cpuid
behaviour, but needs further hypervisor work before the toolstack can sensibly
control these values.

x86_cpuid_copy_to_buffer() is implemented using Xen's regular copy_to_guest
primitives, with an API-compatible memcpy() is used for the libxc half of the
build.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>