I'm afraid that we do not provide sensor for that, however you can still monitor the space used by the virtual machine via its OS. To do so, simply use the native sensors PRTG provides regarding disk space.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.