This plugin stopped working after a VBox upgrade or (more likely) Hudson upgrade.
I can still test the connection in the 'Cloud setup' section, vboxwebsrv logs acknowledge the connection from Hudson and Hudson reports 'Success'. The virtual machine is however not started when it should. It looks like the plugin thinks there are no virtual machines available.
The dropdown box 'Virtual Machine Name' of the Virtual node's configuration page remains empty, and no messages appear in the vboxwebsrv. It appears like the plugin is no longer called from Hudson (or no longer calls the vboxwebsrv), or that some web page form callback handler is not called.
It's probably a trivial fix, but the plugin is now clearly broken, while it still worked about two months ago.