Fix handling of the VBox Manager connection for XPCOM

[tbatchelli]

The way the connection to VBox's XPCOM infrastructure performed in v.v.session using defonce has many issues related to maintaining the underlying vbox process, e.g. if the process VBoxSVC is killed once (server) is invoked, then it becomes impossible to regain access to a new VBoxSVC process from the current vmfest session, requiring a restart of the vmfest process.