[Vivado] Support JSON format XCI files

The XCI parser expects an XML formatted file but:

Note: Starting in Vivado Design Suite version 2018.3, the block design file format has changed from XML to JSON. When you open a block design that uses the older XML schema in Vivado 2018.3 or later, click Save to convert the format from XML to JSON. The following INFO message notifies you of the schema change.