libvirt users - Jan 2019 - unable to list virtualbox domain remotely

Hi dear all,
I have the following issue :
I have a libvirtd intstalled on an ubuntu desktop. The libvirtd is
listening on port 16509 (options listen_tls = 0 and listen_tcp = 1)
On the desktop I have a virtualbox installed with a running VM :
"styx32-dry-run3"
I want to control my VM remotely by using libvirt.
I performed following tests :
1)Local virsh running fine :
 virsh -c vbox:///session list
 Id    Name                           State
----------------------------------------------------
 2     styx32-dry-run3                running

2)Using the tcp port is showing an empty list:
virsh -c vbox+tcp://127.0.0.1:16509/session list
 Id    Name                           State
----------------------------------------------------

3)net-list command works
virsh -c vbox+tcp://127.0.0.1:16509/session net-list
 Name                 State      Autostart     Persistent
----------------------------------------------------------
 vboxnet0             active     no autostart  yes


Can any of you help me please, I really don't understand why my VM is not
showing up in the 2 ) list  .. and I really don't know where to look at :-(

Kind regards
Francois