set_link.py: Bug fix and add error.context

Now vm do not have attribute 'netdev_id'. Change to use
vm.virtnet[0].netdev_id.
Waiting for guest network up again after set_link up.

Also add error.context.

Signed-off-by: Feng Yang <fyang@redhat.com>
set_link: Make root requirement explicit

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>