destroy_nic() requires that NICInfo::private by a PCIDevice pointer,
but then goes on to require that the same pointer matches
VLANClientState::opaque.

That is no longer the case for virtio-net since qdev and wasn't
previously the case for rtl8139, ne2k_pci or eepro100.

Make the situation a lot more clear by maintaining a VLANClientState
pointer in NICInfo.
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>