net: Use hubs for the vlan feature

Stop using the special-case vlan code in net.c.  Instead use the hub net
client to implement the vlan feature.  The next patch will remove vlan
code from net.c completely.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>