e1000: fix link down handling with auto negotiation

Fixes a couple of regression bugs introduced by
b9d03e352cb6b31a66545763f6a1e20c9abf0c2c and related to
auto-negotiation:
-   Auto-negotiation currently sets link up even if it was
    forced down from the monitor.
-   If Auto-negotiation was in progress during migration,
    link will never come up.

As a fix, don't touch NC link_down field at all,
instead add code on receive path to check
guest link status.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
