The new property is meant for management stack (Salt) to set which DVM
template should be used to maintain given VM. Since the DispVM based on
it will be given ultimate control over target VM (qubes.VMShell
service), it should be trusted. The one pointed to by default_dispvm
not necessary is one.
The property defaults to the value from the template (if any), and then
to a global management_dispvm property. By default it is set to None.
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>