core2migration: try to set properties to "default" when possible

Core3 keep information whether property have default value for all the
properties (not only few like netvm or kernel). Try to use this feature
as much as possible.

qubes/core2migration.py

@@ -162,12 +162,23 @@ class Core2Qubes(qubes.Qubes):
                     'template_qid'))]
                 vm_class = AppVM
             # simple attributes
-            for attr in ['installed_by_rpm', 'include_in_backups',
-                    'qrexec_timeout', 'internal', 'label', 'name',
-                    'vcpus', 'memory', 'maxmem', 'default_user',
-                    'debug', 'pci_strictreset', 'mac', 'autostart']:
+            for attr, default in {
+                'installed_by_rpm': 'False',
+                'include_in_backups': 'True',
+                'qrexec_timeout': '60',
+                'internal': 'False',
+                'label': None,
+                'name': None,
+                'vcpus': '2',
+                'memory': '400',
+                'maxmem': '4000',
+                'default_user': 'user',
+                'debug': 'False',
+                'pci_strictreset': 'True',
+                'mac': None,
+                'autostart': 'False'}.items():
                 value = element.get(attr)
-                if value:
+                if value and value != default:
                     kwargs[attr] = value
             # attributes with default value
             for attr in ["kernel", "kernelopts"]: