qemu-options.hx: add missing id=chr0 chardev argument in vhost-user example

In the vhost-user example, a chardev with id chr0 is referenced by the
vhost-user net backend, but the id is not specified in the chardev option.
Signed-off-by: NVincenzo Maffione <v.maffione@gmail.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qemu-options.hx

@@ -2146,7 +2146,7 @@ Example:
 @example
 qemu -m 512 -object memory-backend-file,id=mem,size=512M,mem-path=/hugetlbfs,share=on \
      -numa node,memdev=mem \
-     -chardev socket,path=/path/to/socket \
+     -chardev socket,id=chr0,path=/path/to/socket \
      -netdev type=vhost-user,id=net0,chardev=chr0 \
      -device virtio-net-pci,netdev=net0
 @end example