This allows to specify tight network isolation for a VM, and finally
close one remaining way for leaking traffic around TorVM. Now when VM is
connected to for example TorVM, its DispVMs will be also connected
there.
The new property can be set to:
- default (uses_default_dispvm_netvm=True) - use the same NetVM/ProxyVM as the
calling VM itself - including none it that's the case
- None - DispVMs will be network-isolated
- some NetVM/ProxyVM - will be used, even if calling VM is network-isolated
Closesqubesos/qubes-issues#862
