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: Vincenzo Maffione <v.maffione@gmail.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>