Our code allows snapshots of NVMe based disks which means we create
overlay file with a 'json:{}' pseudo-uri refering to the NVME device.
Our parser code doesn't handle them though. Add the parser and test it
via the XML->json->XML round-trip and reference data.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>