sourceroutethread: fix logging string formatting

Change-Id: I8bc377bacd259fa914cac6fe25424d4fa1c79ef7
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30010
Reviewed-by: Dan Kenigsberg <danken@redhat.com>