'dispvm_allowed' name was confusing, because it suggested being able to
spawn new DispVMs, not being a template for DispVM.
FixesQubesOS/qubes-issues#3047
Clone properties from DispVM template after setting base properties
(qid, name, uuid). This means we can use standard clone_properties()
function. Otherwise various setters may fail - for example
netvm setter require uuid property initialized (for VM lookup in VM
collection).
Also, make dispvm_allowed check more robust - include direct creation of
DispVM, and also check just before VM startup (if property was changed
in the meantime).
FixesQubesOS/qubes-issues#3057
Add auto_cleanup property, which remove DispVM after its shutdown
- this is to unify DispVM handling - less places needing special
handling after DispVM shutdown.
New DispVM inherit all settings from respective AppVM. Move this from
classmethod `DispVM.from_appvm()`, to DispVM constructor. This unify
creating new DispVM with any other VM class.
Notable exception are attached devices - because only one running VM can
have a device attached, this would prevent second DispVM started from
the same AppVM. If one need DispVM with some device attached, one can
create DispVM with auto_cleanup=False. Such DispVM will still not have
persistent storage (as any other DispVM).
Tests included.
QubesOS/qubes-issues#2974
