+ use @var{filename} as an external snapshot, create a temporary
+ file with backing_file=@var{filename}, redirect the write to
+ the temporary one
+@item -l, --load-snapshot=@var{snapshot_param}
+ load an internal snapshot inside @var{filename} and export it
+ as an read-only device, @var{snapshot_param} format is
+ 'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'