remote/machine: add vfio-user property

remote/machine: add vfio-user property

Add vfio-user to x-remote machine. It is a boolean, which indicates if
the machine supports vfio-user protocol. The machine configures the bus
differently vfio-user and multiprocess protocols, so this property
informs it on how to configure the bus.

This property should be short lived. Once vfio-user fully replaces
multiprocess, this property could be removed.

Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 5d51a152a419cbda35d070b8e49b772b60a7230a.1655151679.git.jag.raman@oracle.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>