virtio-net: unbreak self announcement and guest offloads after migration

After commit 019a3edb ("virtio: make
features 64bit wide"). Device's guest_features was actually set after
vdc->load(). This breaks the assumption that device specific load()
function can check guest_features. For virtio-net, self announcement
and guest offloads won't work after migration.

Fixing this by defer them to virtio_net_load() where guest_features
were guaranteed to be set. Other virtio devices looks fine.

Fixes: 019a3edb
       ("virtio: make features 64bit wide")
Cc: qemu-stable@nongnu.org
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>

(cherry picked from commit 1f8828ef)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>