e1000: PHY loopback mode support

The missing of loopback mode prevent the running of self diagnosis
program in guest. This patch adds this support.

After this patch, loopback test of ethtool were passed in guest.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
