"Permission Denied" when trying to use an externally launched virtiofsd with libvirt on Ubuntu LTS 24.04

On Ubuntu LTS 24.04 Server, with a libvirt-managed QEMU virtual machine, I'm trying to use a externally-launched virtiofsd [as documented in the libvirt docs](https://libvirt.org/kbase/virtiofs.html#externally-launched-virtiofsd) , because I need to enable features, such as --posix-acl, not supported by libvirt's virtiofsd config. However, whichever permissions, owner, and group I set the virtiofsd socket, my virtual machine cannot use it, always failing with "Permission Denied":

$ virsh start my-vm
error: Failed to start domain 'my-vm'
error: internal error: process exited while connecting to monitor: 2025-02-07T02:00:44.288580Z qemu-system-x86_64: -chardev socket,id=chr-vu-fs0,path=/run/my-virtiofsd.sock: Failed to connect to '/run/my-virtiofsd.sock': Permission denied

How can I fix it?