libopenflow: Improve adding of port_stats
libopenflow: Improve adding of port_stats

port_stats objects can be added as an easy way of summing the counters.
Previously, this lost the port_no value.  Now, it is retained when it
is the same in both the left and right operand.

VamsikrishnaNallabothu/pox,waltznetworks/pox,xAKLx/pox,denovogroup/pox,chenyuntc/pox,kulawczukmarcin/mypox,VamsikrishnaNallabothu/pox,kavitshah8/SDNDeveloper,waltznetworks/pox,kulawczukmarcin/mypox,andiwundsam/_of_normalize,denovogroup/pox,chenyuntc/pox,PrincetonUniversity/pox,pthien92/sdn,carlye566/IoT-POX,VamsikrishnaNallabothu/pox,MurphyMc/pox,carlye566/IoT-POX,denovogroup/pox,kulawczukmarcin/mypox,diogommartins/pox,adusia/pox,PrincetonUniversity/pox,waltznetworks/pox,xAKLx/pox,kpengboy/pox-exercise,adusia/pox,diogommartins/pox,PrincetonUniversity/pox,andiwundsam/_of_normalize,noxrepo/pox,kavitshah8/SDNDeveloper,carlye566/IoT-POX,pthien92/sdn,noxrepo/pox,jacobq/csci5221-viro-project,adusia/pox,xAKLx/pox,denovogroup/pox,MurphyMc/pox,pthien92/sdn,kpengboy/pox-exercise,noxrepo/pox,xAKLx/pox,waltznetworks/pox,diogommartins/pox,pthien92/sdn,waltznetworks/pox,carlye566/IoT-POX,chenyuntc/pox,denovogroup/pox,MurphyMc/pox,kpengboy/pox-exercise,chenyuntc/pox,diogommartins/pox,andiwundsam/_of_normalize,kpengboy/pox-exercise,andiwundsam/_of_normalize,kavitshah8/SDNDeveloper,pthien92/sdn,VamsikrishnaNallabothu/pox,VamsikrishnaNallabothu/pox,chenyuntc/pox,PrincetonUniversity/pox,xAKLx/pox,MurphyMc/pox,kulawczukmarcin/mypox,PrincetonUniversity/pox,jacobq/csci5221-viro-project,adusia/pox,noxrepo/pox,kpengboy/pox-exercise,adusia/pox,MurphyMc/pox,diogommartins/pox,jacobq/csci5221-viro-project,jacobq/csci5221-viro-project,jacobq/csci5221-viro-project,carlye566/IoT-POX,kavitshah8/SDNDeveloper,kulawczukmarcin/mypox