vhost-user: Fix protocol feature bit conflict

The VHOST_USER_PROTOCOL_F_XEN_MMAP feature bit was defined in
f21e95ee, which has been part of qemu's 8.1.0 release.  However, it
seems it was never added to qemu's code, but it is well possible that it
is already used by different front-ends outside of qemu (i.e., Xen).

VHOST_USER_PROTOCOL_F_SHARED_OBJECT in contrast was added to qemu's code
in 16094766, but never defined in the vhost-user specification.  As a
consequence, both bits were defined to be 17, which cannot work.

Regardless of whether actual code or the specification should take
precedence, F_XEN_MMAP is already part of a qemu release, while
F_SHARED_OBJECT is not.  Therefore, bump the latter to take number 18
instead of 17, and add this to the specification.

Take the opportunity to add at least a little note on the
VhostUserShared structure to the specification.  This structure is
referenced by the new commands introduced in 16094766, but was not
defined.

Fixes: 16094766
       ("vhost-user: add shared_object msg")
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20231016083201.23736-1-hreitz@redhat.com>
Reviewed-by: Emmanouil Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>