libvirtd: systemd: add special target for system shutdown

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