Fix starting time synchronization service

systemd-timesyncd.service isn't enough, for various reasons:
 - it is started too early in the boot process - files in
   /var/run/qubes-services are not yet there
 - by default it does only one shot synchronization, and there is no
   network at that early boot time yet
 - by default use-ntp is set to "no"

So, in addition, enable actual ntp client.

Fixes QubesOS/qubes-issues#3210

vm-systemd/chronyd.service.d/30_qubes.conf

@@ -1,3 +1,4 @@
 [Unit]
-ConditionPathExists=/var/run/qubes-service/ntpd
+ConditionPathExists=|/var/run/qubes-service/clocksync
+ConditionPathExists=|/var/run/qubes-service/ntpd
 After=qubes-sysinit.service