Error on start qemu socket when reinstall ('No KVM' message)

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When reinstall Gnome Boxes from Flatpak can not install any vm, the app says "No KVM" but have KVM in host machine:

```bash
$ sudo kvm-ok
INFO: /dev/kvm exists
KVM acceleration can be used
```

I try find the problem:

```
$ flatpak run org.gnome.Boxes
Ignoring /app/share/osinfo/.ref with missing '.xml' extension

(org.gnome.Boxes:2): GLib-CRITICAL **: 00:12:43.775: g_variant_new_string: assertion 'string != NULL' failed
Ignoring /app/share/osinfo/manifest.json with missing '.xml' extension

(org.gnome.Boxes:2): Boxes-WARNING **: 00:12:47.149: util-app.vala:448: Failed to execute: virsh capabilities
```

```bash
$ git clone https://gitlab.gnome.org/GNOME/gnome-boxes.git
cd gnome-boxes/
$ grep 'No KVM' . -iR
./NEWS:- Fix 'No KVM' warning visibility issues.
./src/vm-configurator.vala: // No KVM-enabled guest caps :( We at least need Qemu
./src/assistant/assistant.vala: show_error ("No KVM!\n");
```

In `./src/assistant/assistant.vala`:

```
private async void setup (string path) {
if (!yield check_libvirt_kvm ()) {
show_error ("No KVM!\n");
return;
}
```

Find the checker:

```
$ grep 'check_libvirt_kvm' . -iR
./src/util-app.vala: public async bool check_libvirt_kvm () {
```

In `./src/util-app.vala`:

```
public async bool check_libvirt_kvm () {
try {
string standard_output;

string[] argv = {"virsh", "capabilities"};

yield exec (argv, null, out standard_output);
int index = standard_output.index_of ("

Start by reproducing `virsh capabilities` inside the Flatpak shell and compare it with the host result. Read `src/assistant/assistant.vala` and `src/util-app.vala`, especially `setup()` and `check_libvirt_kvm()`, then trace how the bundled libvirt socket is started. Done means identifying and resolving the Flatpak startup failure so the KVM check succeeds, with verification of VM creation.

Written by the indexing model from the issue text.