rtl8139: implement 8139cp link status

rtl8139: implement 8139cp link status

Add a link status chang callback and change the link status bit in BMSR
& MSR accordingly. Tested in Linux/Windows guests.

The link status bit of MediaStatus is infered from BasicModeStatus,
they are inverse.

nc.link_down could not be migrated, this patch updates link_down in
rtl8139_post_load() to keep it coincident with real link status.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>