libvirt-guests: run after time-sync.target

When libvirt-guests is configured to start guests on host
boot, it is possible for guests start and read the host
clock before it is synchronized.  Services such as
libvirt-guests that require correct time should use the
Special Passive System Unit time-sync.target

http://www.freedesktop.org/software/systemd/man/systemd.special.html#time-sync.target

tools/libvirt-guests.service.in

 [Unit]
 Description=Suspend Active Libvirt Guests
-After=network.target libvirtd.service
+After=network.target libvirtd.service time-sync.target
 Documentation=man:libvirtd(8)
 Documentation=http://libvirt.org