As the title says and as commented in flathub/org.virt_manager.virt-manager#33, when trying to start a VM with a VirtioFS device configured, this error is displayed:

Previously I also faced this error when I used Virt-Manager installed through apt, and I solved it by installing the virtiofsd package in the host (Debian), but that obviously doesn't work here since both Virt-Manager and QEMU are installed using Flatpak (User Session).

Is there any way to get it working using Flatpak, or in this case do I need to resort to 9P for file sharing?