netinfo.speed: avoid log spam

Some network device drivers, such as infiniband, do not expose their
"speed". In such cases we report speed "0", and fill vdsm.log with a
persistent exception traceback. With this patch, Vdsm no longer seem
surprised by a nic with invalid speed pseudofile.

Thanks for Trey Dockendorf for reporting the issue on
http://lists.ovirt.org/pipermail/users/2014-February/020987.html

Bug-Url: https://bugzilla.redhat.com/1063433
Change-Id: I4b03470139fb383fd03eaa2dc356539eae80d1c0
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24197
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>