follow modern naming of wifi devices

Fedora has forsaken wlan* names, but sampling wifi device still spams
the logs with

  File "/usr/lib64/python2.7/site-packages/vdsm/netinfo.py", line 235, in speed
    s = int(file('/sys/class/net/%s/speed' % dev).read())
IOError: [Errno 22] Invalid argument

Change-Id: Ia0f19269ab3da32af06623d18dcbc1e3b997d013
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17332
Reviewed-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>