netdev: fail when setting up an SRIOV VF if PF is offline

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.