systemd: Only ever restart libvirtd on upgrade

Restarting libvirt-guests.service would result in VMs being
either restarted or stopped, and restarting virtlogd.service
or any of the sockets might result in VMs crashing.

libvirtd, on the other hand, is safe to restart.

debian/libvirt-daemon-system.postinst

@@ -242,7 +242,7 @@ if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-decon
 	if [ -z "${DPKG_ROOT:-}" ] && [ -d /run/systemd/system ]; then
 		systemctl --system daemon-reload >/dev/null || true
 		# We have already called 'reload' for virtlogd and virtlockd earlier if they were active
-		deb-systemd-invoke restart 'libvirt-guests.service' 'virtlockd.service' 'virtlogd.service' >/dev/null || true
+		deb-systemd-invoke start 'libvirt-guests.service' 'virtlockd.service' 'virtlogd.service' >/dev/null || true
 	fi
 fi
 if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-deconfigure" ] || [ "$1" = "abort-remove" ] ; then
@@ -353,7 +353,7 @@ fi
 if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-deconfigure" ] || [ "$1" = "abort-remove" ] ; then
 	if [ -z "${DPKG_ROOT:-}" ] && [ -d /run/systemd/system ]; then
 		systemctl --system daemon-reload >/dev/null || true
-		deb-systemd-invoke restart 'libvirtd-admin.socket' 'libvirtd-ro.socket' 'libvirtd.socket' 'virtlockd-admin.socket' 'virtlockd.socket' 'virtlogd-admin.socket' 'virtlogd.socket' >/dev/null || true
+		deb-systemd-invoke start 'libvirtd-admin.socket' 'libvirtd-ro.socket' 'libvirtd.socket' 'virtlockd-admin.socket' 'virtlockd.socket' 'virtlogd-admin.socket' 'virtlogd.socket' >/dev/null || true
 	fi
 fi
 # End of section copied verbatim