Update service dependencies for modular libvirt daemon

libvirtd.service is the monolithic one, virtxend is modular one. Add
After=virtxend.socket, as it uses socket activation now.

QubesOS/qubes-issues#9402

linux/systemd/qubes-core.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=Qubes Dom0 startup setup
-After=qubes-db-dom0.service libvirtd.service xenconsoled.service qubesd.service qubes-qmemman.service
+After=qubes-db-dom0.service libvirtd.service virtxend.socket xenconsoled.service qubesd.service qubes-qmemman.service
 # Cover legacy init.d script
 
 [Service]