Commit b9dd878f caused a regression in iptables interaction by
logging non-zero status at a higher level than VIR_INFO.  Revert
that portion of the commit, as well as adding a comment explaining
why we check the status ourselves.

Reported by Nehal J Wani.

* src/util/viriptables.c (virIpTablesOnceInit): Undo log regression.
Signed-off-by: NEric Blake <eblake@redhat.com>