vm/systemd: ensure that qubes-sysinit is started with /proc/xen mounted

vm-systemd/qubes-sysinit.service

@@ -2,7 +2,7 @@
 Description=Init Qubes Services settings
 DefaultDependencies=no
 Before=sysinit.target
-After=local-fs.target
+After=local-fs.target proc-xen.mount
 
 [Service]
 Type=oneshot