libx86/CPUID: fix (not just) leaf 7 processing

libx86/CPUID: fix (not just) leaf 7 processing

For one, subleaves within the respective union shouldn't live in
separate sub-structures. And then x86_cpuid_policy_fill_native() should,
as it did originally, iterate over all subleaves here as well as over
all main leaves. Switch to using a "<= MIN()"-based approach similar to
that used in x86_cpuid_copy_to_buffer(). Also follow this for the
extended main leaves then.

Fixes: 1bd2b750537b ("libx86: Fix 32bit stubdom build of x86_cpuid_policy_fill_native()")
Fixes: 97e4ebdcd765 ("x86/CPUID: support leaf 7 subleaf 1 / AVX512_BF16")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>