vm/qubesvm: revert backup_timestamp to '%s' format

Human readable format `str(datetime.datetime)` is a nightmare for Admin
API level communication. Especially setting the property in a format
that it was read was not supported, and handling such format in
untrusted input handling code is a bad idea. Revert to a simple intiger
format.

qubes/backup.py

@@ -819,7 +819,8 @@ class Backup(object):
         # Save date of last backup, only when backup succeeded
         for qid, vm_info in files_to_backup.items():
             if vm_info.vm:
-                vm_info.vm.backup_timestamp = datetime.datetime.now()
+                vm_info.vm.backup_timestamp = \
+                    int(datetime.datetime.now().strftime('%s'))
 
         self.app.save()
 

qubes/tests/vm/qubesvm.py

@@ -513,12 +513,12 @@ class TC_90_QubesVM(QubesVMTestsMixin, qubes.tests.QubesTestCase):
         timestamp = datetime.datetime(2016, 1, 1, 12, 14, 2)
         timestamp_str = timestamp.strftime('%s')
         self.assertPropertyDefaultValue(vm, 'backup_timestamp', None)
-        self.assertPropertyValue(vm, 'backup_timestamp', timestamp,
-            timestamp, timestamp_str)
+        self.assertPropertyValue(vm, 'backup_timestamp', int(timestamp_str),
+            int(timestamp_str), timestamp_str)
         del vm.backup_timestamp
         self.assertPropertyDefaultValue(vm, 'backup_timestamp', None)
         self.assertPropertyValue(vm, 'backup_timestamp', timestamp_str,
-            timestamp)
+            int(timestamp_str))
 
     def test_401_backup_timestamp_invalid(self):
         vm = self.get_vm()

qubes/vm/qubesvm.py

@@ -476,13 +476,9 @@ class QubesVM(qubes.vm.mix.net.NetVMMixin, qubes.vm.BaseVM):
         type=bool, setter=qubes.property.bool,
         doc='If this domain is to be included in default backup.')
 
-    # format got changed from %s to str(datetime.datetime)
     backup_timestamp = qubes.property('backup_timestamp', default=None,
-        setter=(lambda self, prop, value:
-            value if isinstance(value, datetime.datetime) else
-            datetime.datetime.fromtimestamp(int(value))),
-        saver=(lambda self, prop, value: value.strftime('%s')),
-        doc='FIXME')
+        type=int,
+        doc='Time of last backup of the qube, in seconds since unix epoch')
 
     default_dispvm = qubes.VMProperty('default_dispvm',
         load_stage=4,