app: add missing load_stage=3 to global properties

Global properties should be loaded in stage 3, mark them as such.
Otherwise they are not loaded at all.
This applies to stats_interval and check_updates_vm. Others were
correct.

Fixes QubesOS/qubes-issues#4856

qubes/app.py

@@ -770,6 +770,7 @@ class Qubes(qubes.PropertyHolder):
         doc='''Default time in seconds for VM shutdown to complete''')
 
     stats_interval = qubes.property('stats_interval',
+        load_stage=3,
         default=3,
         type=int,
         doc='Interval in seconds for VM stats reporting (memory, CPU usage)')
@@ -777,6 +778,7 @@ class Qubes(qubes.PropertyHolder):
     # TODO #1637 #892
     check_updates_vm = qubes.property('check_updates_vm',
         type=bool, setter=qubes.property.bool,
+        load_stage=3,
         default=True,
         doc='check for updates inside qubes')