qemu-doc: Get rid of "vlan=X" example in the documentation

The vlan concept is marked as deprecated, so we should not use
this for examples in the documentation anymore.
Signed-off-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>

qemu-options.hx

@@ -2242,8 +2242,8 @@ qemu-system-i386 linux.img -net nic -net tap
 #launch a QEMU instance with two NICs, each one connected
 #to a TAP device
 qemu-system-i386 linux.img \
-                 -net nic,vlan=0 -net tap,vlan=0,ifname=tap0 \
-                 -net nic,vlan=1 -net tap,vlan=1,ifname=tap1
+        -netdev tap,id=nd0,ifname=tap0 -device e1000,netdev=nd0 \
+        -netdev tap,id=nd1,ifname=tap1 -device rtl8139,netdev=nd1
 @end example
 
 @example