target-i386: Reject invalid CPU feature names on the command-line

Instead of simply printing a warning, report an error when invalid CPU
options are provided on the CPU model string.

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