Fix networks in LUInstanceSetParams()

Params passed in _CreateNewNic() are not yet evaluated and include
the value passed by user for the network. A lookup must be done first
in order to find the corresponding network UUID which should be stored
in the newly created NIC object.

Params passed in _ApplyNicMods() include the value passed by user
for the network. Thus a lookup must be done first in order to find
the corresponding network UUID. Then compare it with the existing
value and act accordingly.

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