libvirtd: systemd: add special target for system shutdown

It is already discussed in "[RFC] daemon: remove hardcode dep on libvirt-guests" [1].

Mgmt can use means to save/restore domains on system shutdown/boot other than
libvirt-guests.service. Thus we need to specify appropriate ordering dependency between
libvirtd, domains and save/restore service. This patch takes approach suggested
in RFC and introduces a systemd target, so that ordering can be built next way:

libvirtd -> domain -> virt-guest-shutdown.target -> save-restore.service.

This way domains are decoupled from specific shutdown service via intermediate
target.

[1] https://www.redhat.com/archives/libvir-list/2016-September/msg01353.html

daemon/Makefile.am

@@ -67,6 +67,7 @@ EXTRA_DIST =						\
 	libvirt.rules					\
 	libvirtd.sasl					\
 	libvirtd.service.in				\
+	virt-guest-shutdown.target		\
 	libvirtd.sysconf				\
 	libvirtd.sysctl					\
 	libvirtd.aug                                    \
@@ -456,8 +457,11 @@ install-init-systemd: install-sysconfig libvirtd.service
 	$(MKDIR_P) $(DESTDIR)$(SYSTEMD_UNIT_DIR)
 	$(INSTALL_DATA) libvirtd.service \
 	  $(DESTDIR)$(SYSTEMD_UNIT_DIR)/libvirtd.service
+	$(INSTALL_DATA) virt-guest-shutdown.target \
+	  $(DESTDIR)$(SYSTEMD_UNIT_DIR)/virt-guest-shutdown.target
 
 uninstall-init-systemd: uninstall-sysconfig
+	rm -f $(DESTDIR)$(SYSTEMD_UNIT_DIR)/virt-guest-shutdown.target
 	rm -f $(DESTDIR)$(SYSTEMD_UNIT_DIR)/libvirtd.service
 	rmdir $(DESTDIR)$(SYSTEMD_UNIT_DIR) || :
 else ! LIBVIRT_INIT_SCRIPT_SYSTEMD

daemon/virt-guest-shutdown.target

@@ -0,0 +1,3 @@
+[Unit]
+Description=Libvirt guests shutdown
+Documentation=http://libvirt.org