docs: add vfio-user documentation

Add some basic documentation on vfio-user usage.

Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250625193012.2316242-19-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

docs/system/device-emulation.rst

@@ -85,6 +85,7 @@ Emulated Devices
    devices/can.rst
    devices/ccid.rst
    devices/cxl.rst
+   devices/vfio-user.rst
    devices/ivshmem.rst
    devices/ivshmem-flat.rst
    devices/keyboard.rst

docs/system/devices/vfio-user.rst

@@ -0,0 +1,26 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+=========
+vfio-user
+=========
+
+QEMU includes a ``vfio-user`` client. The ``vfio-user`` specification allows for
+implementing (PCI) devices in userspace outside of QEMU; it is similar to
+``vhost-user`` in this respect (see :doc:`vhost-user`), but can emulate arbitrary
+PCI devices, not just ``virtio``. Whereas ``vfio`` is handled by the host
+kernel, ``vfio-user``, while similar in implementation, is handled entirely in
+userspace.
+
+For example, SPDK includes a virtual PCI NVMe controller implementation; by
+setting up a ``vfio-user`` UNIX socket between QEMU and SPDK, a VM can send NVMe
+I/O to the SPDK process.
+
+Presuming a suitable ``vfio-user`` server has opened a socket at
+``/tmp/vfio-user.sock``, a device can be configured with for example:
+
+.. code-block:: console
+
+-device '{"driver": "vfio-user-pci","socket": {"path": "/tmp/vfio-user.sock", "type": "unix"}}'
+
+See `libvfio-user <https://github.com/nutanix/libvfio-user/>`_ for further
+information.