vfio-user: introduce vfio-user protocol specification

This patch introduces the vfio-user protocol specification (formerly
known as VFIO-over-socket), which is designed to allow devices to be
emulated outside QEMU, in a separate process. vfio-user reuses the
existing VFIO defines, structs and concepts.

It has been earlier discussed as an RFC in:
"RFC: use VFIO over a UNIX domain socket to implement device offloading"

Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>
Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250625193012.2316242-20-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

MAINTAINERS

@@ -4246,7 +4246,6 @@ F: hw/remote/proxy-memory-listener.c
 F: include/hw/remote/proxy-memory-listener.h
 F: hw/remote/iohub.c
 F: include/hw/remote/iohub.h
-F: subprojects/libvfio-user
 F: hw/remote/vfio-user-obj.c
 F: include/hw/remote/vfio-user-obj.h
 F: hw/remote/iommu.c
@@ -4257,6 +4256,8 @@ VFIO-USER:
 M: John Levon <john.levon@nutanix.com>
 M: Thanos Makatos <thanos.makatos@nutanix.com>
 S: Supported
+F: docs/interop/vfio-user.rst
+F: docs/system/devices/vfio-user.rst
 F: hw/vfio-user/*
 F: include/hw/vfio-user/*
 F: subprojects/libvfio-user

docs/interop/index.rst

@@ -25,6 +25,7 @@ are useful for making QEMU interoperate with other software.
    qemu-ga-ref
    qemu-qmp-ref
    qemu-storage-daemon-qmp-ref
+   vfio-user
    vhost-user
    vhost-user-gpu
    vhost-vdpa