x86/CPUID: shrink max_{,sub}leaf fields according to actual leaf contents

x86/CPUID: shrink max_{,sub}leaf fields according to actual leaf contents

Zapping leaf data for out of range leaves is just one half of it: To
avoid guests (bogusly or worse) inferring information from mere leaf
presence, also shrink maximum indicators such that the respective
trailing entry is not all blank (unless of course it's the initial
subleaf of a leaf that's not the final one).

This is also in preparation of bumping the maximum basic leaf we
support, to ensure guests not getting exposed related features won't
observe a change in behavior.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>