This attribute indicates the number of virtual links in use on this
AQM. Its maximum value is the value of operational attribute
maxVirtualLinks. Since each port uses one virtual link, if
maxVirtualLinks has value sameNumberAsPorts then
virtualLinkUsage will be 1 for 2pOC3MmAtm2 and
2pOC3SmAtm2 cards, and 3 for 3pDS3Atm2 and 3pE3Atm2
cards.