utils/block: catch an exception when talking to disconnected qubesdb

This can happen for example when domain disappeared in the meantime.

core/qubesutils.py

@@ -241,7 +241,12 @@ def block_list_vm(vm, system_disks = False):
 
     devices_list = {}
 
+    try:
         untrusted_devices = vm.qdb.multiread('/qubes-block-devices/')
+    except DisconnectedError:
+        # FIXME: find some better way to do that
+        vm._qdb_connection = None
+        return {}
 
     def get_dev_item(dev, item):
         return untrusted_devices.get(