The legacy_name is useless now, better help
information is provided by description field of property.

Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>