slapos_cloud: force source_project accessors when using SlapOSVirtualMasterSourceProjectConstraint

master/bt5/slapos_cloud/PropertySheetTemplateItem/portal_property_sheets/SlapOSVirtualMasterSourceProjectConstraint/source_project_category.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="Category Property" module="erp5.portal_type"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>description</string> </key>
+            <value>
+              <none/>
+            </value>
+        </item>
+        <item>
+            <key> <string>id</string> </key>
+            <value> <string>source_project_category</string> </value>
+        </item>
+        <item>
+            <key> <string>portal_type</string> </key>
+            <value> <string>Category Property</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>