target-i386: Sanitize AMD's ext2_features at realize time
When CPU properties are implemented, ext2_features may change
between object_new(CPU) and cpu_realize_fn(). Sanitizing
ext2_features for AMD based CPU at realize() time will keep
current behavior after CPU features are converted to properties.
Signed-off-by: Igor Mammedov <imammedo@redhat.com> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com> Signed-off-by: Andreas Färber <afaerber@suse.de>