libnl: non-existing link returns exception instead of None

Because of backward compatibility we use two different functions for
getting the link in _get_link(). In case of non-existing device,
rtnl_link_get_by_name() returns None, rtnl_link_get_kernel() raises
NLE_NODEV exception.

To make _get_link() function consistent, we return link as None in the
case of NLE_NODEV exception.

Change-Id: I4cf5c7d1cb89bb77402b6e3d23fb0e76f06e122e
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34690
Reviewed-by: Dan Kenigsberg <danken@redhat.com>