Helper function for constructing AsyncInitable object.
This is similar to new_valist but also initializes the object asynchronously.
When the initialization is finished,
callback will be called. You can then call
new_async.end to get the new object and check for any errors.
the name of the first property, followed by the value, and other property value pairs, and ended by null .
The var args list generated from
the I/O priority of the operation
optional Cancellable object, null to ignore.
a TaskReadyCallback to call when the initialization is finished
the data to pass to callback function