netdev: fail when setting up an SRIOV VF if PF is offline
    
    If an SRIOV PF is offline, the kernel won't complain if you set the
    mac address and vlan tag for a VF via this PF, and it will even let
    you assign the VF to a guest using PCI device assignment or macvtap
    passthrough. But in this case (the PF isn't online), the device won't
    be usable in the guest.
    
    Silently setting the PF online would solve the connectivity problem,
    but as pointed out by Dan Berrange, when an interface is set online
    with no associated config, the kernel will by default turn on IPv6
    autoconf, which could create unexpected security problems for the
    host. For this reason, this patch instead logs an error and fails the
    operation.
    
    This resolves: https://bugzilla.redhat.com/show_bug.cgi?id=893738
    
    Originally filed against RHEL6, but present in every version of
    libvirt until today.