Add vhost-user-backend

Add vhost-user-backend

Create a vhost-user-backend object that holds a connection to a
vhost-user backend (or "slave" process) and can be referenced from
virtio devices that support it. See later patches for input & gpu
usage.

Note: a previous iteration of this object made it user-creatable, and
allowed managed sub-process spawning, but that has been dropped for
now.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190503130034.24916-4-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>