i386: rewrite way CPUID index is validated

Change the nested if statements into a flat format, to make
it clearer what validation / capping is being performed on
different CPUID index values.

NB this changes behaviour when "index > env->cpuid_xlevel2".
This won't have any guest-visible effect because no there is
no CPUID[0xC0000001] feature supported by TCG, and KVM code
will never call cpu_x86_cpuid() with such an index value.

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20170509132736.10071-2-berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
