net/passt: Implement vhost-user backend support

This commit adds support for the vhost-user interface to the passt
network backend, enabling high-performance, accelerated networking for
guests using passt.

The passt backend can now operate in a vhost-user mode, where it
communicates with the guest's virtio-net device over a socket pair
using the vhost-user protocol. This offloads the datapath from the
main QEMU loop, significantly improving network performance.

When the vhost-user=on option is used with -netdev passt, the new
vhost initialization path is taken instead of the standard
stream-based connection.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

docs/system/devices/net.rst

@@ -104,7 +104,7 @@ To use the passt backend interface
 
 There is no need to start the daemon as QEMU will do it for you.
 
-passt is started in the socket-based mode.
+By default, passt will be started in the socket-based mode.
 
 .. parsed-literal::
    |qemu_system| [...OPTIONS...] -nic passt
@@ -128,6 +128,16 @@ passt is started in the socket-based mode.
    virtio0: index=0,type=nic,model=virtio-net-pci,macaddr=9a:2b:2c:2d:2e:2f
     \ netdev0: index=0,type=passt,stream,connected to pid 25428
 
+To use the vhost-based interface, add the ``vhost-user=on`` parameter and
+select the virtio-net device:
+
+.. parsed-literal::
+   |qemu_system| [...OPTIONS...] -nic passt,model=virtio,vhost-user=on
+
+   (qemu) info network
+   virtio-net-pci.0: index=0,type=nic,model=virtio-net-pci,macaddr=52:54:00:12:34:56
+    \ #net006: index=0,type=passt,vhost-user,connected to pid 25731
+
 To use socket based passt interface:
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^