Can't launch VM because of "util-app.vala:442: Failed to execute: virsh capabilities" error

Steps to reproduce:
1. Have Debian 11 installed with Flatpak
2. Install GNOME Boxes with `flatpak install flathub org.gnome.Boxes`
3. Run GNOME Boxes with `flatpak run org.gnome.Boxes`
4. Try to install some OS - click "+", select OS (*Debian 11 netinst* in my example), wait for download, wait for *New Virtual Machine* window to appear, try to click *Create*

Expected results - VM is created and runs well.

Actual results - unable to create VM, terminal shows the following text:
```
$ flatpak run org.gnome.Boxes
Gtk-Message: 20:24:47.931: Failed to load module "appmenu-gtk-module"
Gtk-Message: 20:24:47.931: Failed to load module "canberra-gtk-module"

(gnome-boxes:2): GLib-CRITICAL **: 20:24:53.461: g_variant_new_string: assertion 'string != NULL' failed
Ignoring /app/share/osinfo/.ref with missing '.xml' extension
Ignoring /app/share/osinfo/manifest.json with missing '.xml' extension

(gnome-boxes:2): Boxes-WARNING **: 20:25:00.164: util-app.vala:442: Failed to execute: virsh capabilities
Ignoring /app/share/osinfo/.ref with missing '.xml' extension
Ignoring /app/share/osinfo/manifest.json with missing '.xml' extension

(gnome-boxes:2): Boxes-WARNING **: 20:28:47.039: util-app.vala:442: Failed to execute: virsh capabilities
```

Note: I can run both VirtualBox 6.1 and QEMU/KVM on this machine with Intel CPU.