target-i386: Isolate KVM-specific code on CPU feature filtering logic

This will allow us to re-use the feature filtering logic (and the
check/enforce flag logic) for TCG.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
