Make virtual vector layers to support direct access only

Fixes opengisch/qfieldsync#514

layer.py

@@ -300,7 +300,9 @@ def set_relationship_maximum_visible(
 
     @property
     def default_action(self):
-        if self.is_file:
+        if self.is_virtual:
+            return SyncAction.NO_ACTION
+        elif self.is_file:
             return SyncAction.COPY
         elif not self.is_supported:
             return SyncAction.REMOVE
@@ -325,10 +327,27 @@ def is_cloud_configured(self):
     def is_file(self):
         return os.path.isfile(self.filename)
 
+    @property
+    def is_virtual(self):
+        return (
+            self.layer.dataProvider() and self.layer.dataProvider().name() == "virtual"
+        )
+
     @property
     def available_actions(self):
         actions = list()
 
+        if self.is_virtual:
+            actions.append(
+                (
+                    SyncAction.NO_ACTION,
+                    QCoreApplication.translate(
+                        "LayerAction", "Directly access data source"
+                    ),
+                )
+            )
+            return actions
+
         if self.is_file and not self.is_localized_path:
             actions.append(
                 (SyncAction.COPY, QCoreApplication.translate("LayerAction", "Copy"))
@@ -372,6 +391,17 @@ def available_actions(self):
     def available_cloud_actions(self):
         actions = []
 
+        if self.is_virtual:
+            actions.append(
+                (
+                    SyncAction.NO_ACTION,
+                    QCoreApplication.translate(
+                        "LayerAction", "Directly access data source"
+                    ),
+                )
+            )
+            return actions
+
         if self.layer.type() == QgsMapLayer.VectorLayer:
             # all vector layers can be converted for offline editting
             actions.append(