virNetDevGetLinkInfo: Don't report link speed if NIC's not up

virNetDevGetLinkInfo: Don't report link speed if NIC's not up

The kernel's more broken than one would think. Various drivers report
various (usually spurious) values if the interface is in other state
than 'up' . While on some we experience -EINVAL when read()-ing the
speed sysfs file, with other drivers we might get anything from 0 to
UINT_MAX. If that's the case it's better to not report link speed.
Well, the interface is not up anyway.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>