virtio-devices, vmm: vhost: net: Add client mode support

Adding the support for an OVS vhost-user backend to connect as the
vhost-user client. This means we introduce with this patch a new
option to our `--net` parameter. This option is called 'server' in order
to ask the VMM to run as the server for the vhost-user socket.

Fixes #1745

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vmm/src/api/openapi/cloud-hypervisor.yaml

@@ -712,6 +712,9 @@ components:
           default: false
         vhost_socket:
           type: string
+        vhost_mode:
+          type: string
+          default: "client"
         id:
           type: string
         fd: