net: make network client name unique

assign_name() creates a name MODEL.NUM, where MODEL is the client's model,
and NUM is the number of MODELs that already exist.

Markus added NIC naming for non-VLAN clients in commit 53e51d85.
commit d33d93b2 incorrectly added a judgement of net-hub. It caused
net clients created with -netdev get same names.

eg:
 # qemu-upstream -device virtio-net-pci,netdev=h1 -netdev tap,id=h1 \
                    -device virtio-net-pci,netdev=h2 -netdev tap,id=h2 ..
 (qemu) info network
 virtio-net-pci.0: index=0,type=nic,model=virtio-net-pci,macaddr=52:54:00:12:34:56
  \ h1: index=0,type=tap,ifname=tap0,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown
 virtio-net-pci.0: index=0,type=nic,model=virtio-net-pci,macaddr=52:54:00:12:34:57
  \ h2: index=0,type=tap,ifname=tap1,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown

This patch removed the check of nic-hub, and created unique names for
all net clients that have same model.

v2: update commitlog & comments

Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
