libvirt-guests: avoid sourcing failure

* tools/libvirt-guests.init.in (libvirtd): Skip function library
if it is not present.
Suggested by Guido Günther.

tools/libvirt-guests.init.in

@@ -29,7 +29,8 @@ localstatedir=@localstatedir@
 libvirtd=@sbindir@/libvirtd
 
 # Source function library.
-. "$sysconfdir"/rc.d/init.d/functions
+test ! -r "$sysconfdir"/rc.d/init.d/functions ||
+  . "$sysconfdir"/rc.d/init.d/functions
 
 URIS=default
 ON_BOOT=start