Fail early on missing necessary preload service

Occurs when the global max preload feature, set on dom0, with the
default-dispvm being an unsupported template, or old ISOs, old
templates, downgraded templates or packages. When setting the dom0
feature or when switching the default-dispvm, there is no supported-rpc
validation.

Without this validation, it tries to preload and fails on startup
validation. Raise exception early.

For: QubesOS/qubes-issues#1512

Author: ben-grande

qubes/vm/mix/dvmtemplate.py

@@ -103,8 +103,7 @@ def on_feature_pre_set_preload_dispvm_max(
             raise qubes.exc.QubesValueError("Qube does not support qrexec")
 
         service = "qubes.WaitForRunningSystem"
-        supported_service = "supported-rpc." + service
-        if not self.features.check_with_template(supported_service, False):
+        if not self.supports_preload():
             raise qubes.exc.QubesValueError(
                 "Qube does not support the RPC '%s'" % service
             )
@@ -265,6 +264,13 @@ async def on_domain_preload_dispvm_used(
             event_log += " because %s" % str(kwargs.get("reason"))
         self.log.info(event_log)
 
+        service = "qubes.WaitForRunningSystem"
+        if not self.supports_preload():
+            raise qubes.exc.QubesValueError(
+                "Qube does not support the RPC '%s' but tried to preload, "
+                "check if template is outdated" % service
+            )
+
         if event == "autostart":
             self.remove_preload_excess(0)
         elif not self.can_preload():
@@ -406,3 +412,12 @@ def remove_preload_excess(self, max_preload: Optional[int] = None) -> None:
                 if unwanted_disp in self.app.domains:
                     dispvm = self.app.domains[unwanted_disp]
                     asyncio.ensure_future(dispvm.cleanup())
+
+    def supports_preload(self) -> bool:
+        """Returns ``True`` if the necessary RPC is supported."""
+        assert isinstance(self, qubes.vm.BaseVM)
+        service = "qubes.WaitForRunningSystem"
+        supported_service = "supported-rpc." + service
+        if self.features.check_with_template(supported_service, False):
+            return True
+        return False