core: use vm.absolute_path to parse paths in qubes.xml

This makes easier to handle some corner cases.

qubesos/qubes-issues#1124
Reported by @doncohen, thanks @wyory for providing more details.

appmenus-scripts/qubes-core-appmenus.py

@@ -51,22 +51,21 @@ system_path['appmenu_remove_cmd'] = \
 def QubesVm_get_attrs_config(self, attrs):
     attrs["appmenus_templates_dir"] = {
         "func": lambda x:
-            os.path.join(self.dir_path, vm_files["appmenus_templates_subdir"])
+            self.absolute_path(vm_files["appmenus_templates_subdir"], None)
             if self.updateable else
             (self.template.appmenus_templates_dir if self.template is not None
              else None)
     }
     attrs["appmenus_template_icons_dir"] = {
         "func": lambda x:
-            os.path.join(self.dir_path,
-                         vm_files["appmenus_template_icons_subdir"])
+            self.absolute_path(vm_files["appmenus_template_icons_subdir"], None)
             if self.updateable else
             (self.template.appmenus_template_icons_dir
              if self.template is not None else None)
     }
     attrs["appmenus_icons_dir"] = {
         "func": lambda x:
-            os.path.join(self.dir_path, vm_files["appmenus_icons_subdir"])}
+            self.absolute_path(vm_files["appmenus_icons_subdir"], None)}
     return attrs