Fix networks in _PrepareNicModifications()

Passing --net 0:add,ip=5.5.5.5 failed due to a reference
of a non initialized variable (new_net_obj). Reorder the checks
and add some comments to be readable.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Guido Trotter <ultrotter@google.com>