libnl: workaround for _rtnl_link_get_kernel on el6

If we pass just a link's name to rtnl_link_get_kernel(), el6's kernel
is not able to return link object. For this case we created
a workaround with cache and rtnl_link_get_by_name().

Change-Id: If16ea9321669256a1881dccdaed10dda7e6b7379
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35518
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Tested-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>