Documentation: Add documentation for VDUSE Address Space IDs

Documentation: Add documentation for VDUSE Address Space IDs

Address Space IDs allows the VDUSE framework to support devices able to
expose different virtqueues to different part of the drivers.  For
example, to let QEMU handle the net device control virtqueue, so QEMU
always knows the state of the device like mac address or number of
queues enabled, while leaving the dataplane passthrough to the guest
intact.  This enables live migration.

Expands the VDUSE documentation to explain how to use the new ioctls or
the new struct members of old ioctls.

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260119143306.1818855-14-eperezma@redhat.com>