'dispvm_allowed' name was confusing, because it suggested being able to spawn new DispVMs, not being a template for DispVM. Fixes QubesOS/qubes-issues#3047
Not on what VM can ask for (which may be later overriden by target= option). Fixes QubesOS/qubes-issues#3006
Output possible connections between VMs in form of dot file. Fixes QubesOS/qubes-issues#2873