core: improve handling dead domains when talking to QubesDB daemon

Provide vm.refresh(), which will force to reconnect do QubesDB daemon,
and also get new libvirt object (including new ID, if any). Use this
method whenever QubesDB call returns DisconnectedError exception. Also
raise that exception when someone is trying to talk to not running
QubesDB - instead of returning None.

core-modules/000QubesVm.py

@@ -654,9 +654,8 @@ class QubesVm(object):
     @property
     def qdb(self):
         if self._qdb_connection is None:
-            if self.is_running():
-                from qubes.qdb import QubesDB
-                self._qdb_connection = QubesDB(self.name)
+            from qubes.qdb import QubesDB
+            self._qdb_connection = QubesDB(self.name)
         return self._qdb_connection
 
     @property
@@ -705,6 +704,10 @@ class QubesVm(object):
         # obsoleted
         return self.uuid
 
+    def refresh(self):
+        self._libvirt_domain = None
+        self._qdb_connection = None
+
     def get_mem(self):
         if dry_run:
             return 666

core/qubesutils.py

@@ -244,8 +244,7 @@ def block_list_vm(vm, system_disks = False):
     try:
         untrusted_devices = vm.qdb.multiread('/qubes-block-devices/')
     except DisconnectedError:
-        # FIXME: find some better way to do that
-        vm._qdb_connection = None
+        vm.refresh()
         return {}
 
     def get_dev_item(dev, item):