Report multiple IPv4 addresses per device if available

A future version of python-ethtool is going to expose
ethtool.etherinfo.get_ipv4_addresses() method, reporting multiple
addresses of a single device. Use this method if available.

Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Change-Id: I77bfd63874134b8d0c626c6d179643c2ec781b44