netinfo: report "speed" of underlying device as that of vlan device

netinfo.py was reporting speed 0 for the vlan devices in the system
This patch makes that vlan devices, just like nics and bonds, will
have a speed reported, that one of their underlying device.

Change-Id: Iee7f1ea72c77bb628640c1b2b27aac51811c8b0d
Bug-Url: https://bugzilla.redhat.com/1062617
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24990
Reviewed-by: Dan Kenigsberg <danken@redhat.com>