e1000: emulate auto-negotiation during external link status change

This patch emulates auto-negotiation when the network link status
is modified externally (i.e. via "set_link <id> off/on").

Also, a couple of cleanup items:
  - unset PHY status reg. AUTONEG_COMPLETE during link_down()
  - set PHY status reg. AUTONEG_COMPLETE during autoneg_timer() only
    if we actually brought the link up.
  - group all checks for "can we, and should we autonegotiate?"
    together for more clarity.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>


