The current syntax uses a pair of args

   -net nic,macaddr=52:54:00:56:6c:55,vlan=3,model=pcnet,name=pcnet.0
   -net user,vlan=3,name=user.0

The new syntax does not  need the vlan craziness anymore, and
so has a simplified pair of args

   -netdev user,id=user.0
   -device pcnet,netdev=user.0,id=pcnet.0,mac=52:54:00:56:6c:55,addr=<PCI SLOT>