libvirt-guests: Disable shutdown timeout

Since we can't know at service start how many VMs will be running we
can't calculate an apropriate shutdown timeout. So instead of killing
off the service just let it use it's own internal timeout mechanism.

References:
    http://bugs.debian.org/803714
    https://bugzilla.redhat.com/show_bug.cgi?id=1195544

tools/libvirt-guests.service.in

@@ -13,6 +13,7 @@ ExecStop=@libexecdir@/libvirt-guests.sh stop
 Type=oneshot
 RemainAfterExit=yes
 StandardOutput=journal+console
+TimeoutStopSec=0
 
 [Install]
 WantedBy=multi-user.target