Enable disposable preload service via preset

Fixes: QubesOS/qubes-issues#10144
For: QubesOS/qubes-issues#1512

Author: ben-grande

system-config/75-qubes-dom0.preset

@@ -67,6 +67,7 @@ enable qubes-qmemman.service
 enable qubes-suspend.service
 enable qubes-setupdvm.service
 enable qubes-qrexec-policy-daemon.service
+enable qubes-preload-dispvm.service
 enable qubesd.service
 enable anti-evil-maid-unseal.service
 enable anti-evil-maid-check-mount-devs.service