qemu: fix index error when clean up vport profile

qemu: fix index error when clean up vport profile

1. 'last_good_net' indicates the index of last successfully configured
net. so def->nets[last_good_net] should also be clean up if error occurs.

2. if error occurs in 'virNetDevMacVLanVPortProfileRegisterCallback'
(second 'goto err_exit' in loop), we should also do
'virNetDevVPortProfileDisassociate' cleanup for the
'virNetDevVPortProfileAssociate'(first code block in loop). So we should
consider the net is successfully configured after first code block in
loop finishes.

Signed-off-by: Huanle Han <hanxueluo@gmail.com>