target-i386: Convert 'check' and 'enforce' to static properties

* additionally convert check_cpuid & enforce_cpuid to bool and make them
  members of X86CPU
* make 'enforce' feature independent from 'check'

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>