libvirt つかってるけど、qemu.conf にこんな設定がある。
なんかの参考になれば...

# VNC is configured to listen on 127.0.0.1 by default.
# To make it listen on all public interfaces, uncomment
# this next option.
#
# NB, strong recommendation to enable TLS + x509 certificate
# verification when allowing public access
#
# vnc_listen = "0.0.0.0"