multiple headless VMs: VBox daemons stop without waiting for the last VM to shut down

[Régis Desgroppes]

Hi,
For continuous integration purposes, I'm running multiple concurrent headless VMs thanks to the vboxapi python bindings. This is done on demand to release resources as soon as possible.
What I observe:

• everything goes fine when the first started VM is also the last to shut down.
  
• but, if the first started VM shuts down while other VMs are still running, all remaining sessions abort (vboxapi raises a xpcom.Exception: 0x80004004, Operation aborted, NS_ERROR_ABORT) without any sort of notification to running VMs. There's even nothing in any of the VBox.logs (see attached one).

What I guess is that the first started VM "owns" the daemons so, as a workaround, I currently launch one of VBox daemons manually (/usr/lib/virtualbox/VBoxSVC --daemonize) in the hope the daemons are then owned by no peculiar VM. It still works fine after a few days.
Is there a better/recommended way to handle this?
Did I guess well and -if yes- would it be possible to replace the ownership by a reference counting?
Many thanks,
Régis

[Régis Desgroppes]

prematurely ended VM log