qubes: allow passing name of class to app.add_new_vm

This will allow more flexible API usage, especially when using mgmt API
- we need to use VM type as string there.
We don't lose any flexibility here - VM class names needs to be uniquely
identified by a string (used in qubes.xml) anyway.

qubes/app.py

@@ -924,6 +924,8 @@ class Qubes(qubes.PropertyHolder):
         if qid is None:
             qid = self.domains.get_new_unused_qid()
 
+        if isinstance(cls, str):
+            cls = self.get_vm_class(cls)
         # handle default template; specifically allow template=None (do not
         # override it with default template)
         if 'template' not in kwargs and hasattr(cls, 'template'):