core: fix handling vm.start_time for just shutdown VM

That xenstore entry may be already removed even when libvirt still
reports the VM as running.

Fix QubesOS/qubes-issues#2127

core-modules/000QubesVm.py

@@ -967,7 +967,7 @@ class QubesVm(object):
         uuid = self.uuid
 
         start_time = vmm.xs.read('', "/vm/%s/start_time" % str(uuid))
-        if start_time != '':
+        if start_time:
             return datetime.datetime.fromtimestamp(float(start_time))
         else:
             return None