qdev: Hide "ptr" properties from users

Users can't set them, so qdev_device_help() shouldn't list them.  Fix
that.  Also make qdev_prop_parse() hide them instead of printing a
meaningless "has no parser" error message.

Their value means nothing to users, so qdev_print_props() shouldn't
print it.  Fix by removing their print method.

Their only use is dirty hacks.  Document that.
