x86/cpuid: Alter the policy logic for leaf 0xb to be multi-invocation

x86/cpuid: Alter the policy logic for leaf 0xb to be multi-invocation

The new data lives in the .topo union, rather than being treated as a single
leaf in the basic union.

While adjusting cpuid_policy, pad .basic to CPUID_GUEST_NR_BASIC for the
benefit of people extending the number of leaves in the future.

Host data is scanned when filling in the raw policy, but Xen still discards
any toolstack settings for now.

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