* Returns the VolumeManager instance asynchronously. If it is not created or
* under initialization, it will waits for the finish of the initialization.
* @param {function(VolumeManager)} callback Called with the VolumeManager
* Returns the VolumeManager instance asynchronously. If it is not created or
* under initialization, it will waits for the finish of the initialization.
* @param {function(VolumeManager)} callback Called with the VolumeManager