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
691a6f4d8c