Update vhost_sock_dir ML2 setting

The `vhost_sock_dir` ML2 ovn setting specifies the location
where Libvirt will place vhost-user unix sockets consumed
by ovs.

Right now, attempting to use vhost-user (dpdk) ports fails
since `/run/libvirt-vhost-user` doesn't exist:

```
libvirt.libvirtError: internal error: process exited while connecting
to monitor: 2025-08-08T08:09:05.144128Z qemu-system-x86_64:
-chardev socket,id=charnet0,path=/run/libvirt-vhost-user/vhu6a1804f8-e7,server=on:
Failed to bind socket to /run/libvirt-vhost-user/vhu6a1804f8-e7: No such file or directory
```

We'll configure it to use the Libvirt socket dir instead:
`/var/snap/openstack-hypervisor/common/run/libvirt`
c315284fc5/templates/libvirtd.conf.j2 (L115)

Change-Id: Id588b03de7b805f9ddfd0753d2b262c4df1cbfc4
Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>

charms/neutron-k8s/src/charm.py

@@ -359,7 +359,7 @@ class OVNContext(sunbeam_ctxts.ConfigContext):
             "dhcp_default_lease_time": "43200",
             "ovn_dhcp4_global_options": "",
             "ovn_dhcp6_global_options": "",
-            "vhost_sock_dir": "/run/libvirt-vhost-user",
+            "vhost_sock_dir": "/var/snap/openstack-hypervisor/common/run/libvirt",
             "ovn_key": "/etc/neutron/plugins/ml2/key_host",
             "ovn_cert": "/etc/neutron/plugins/ml2/cert_host",
             "ovn_ca_cert": "/etc/neutron/plugins/ml2/neutron-ovn.crt",