vhost-user: Send VHOST_RESET_OWNER on vhost stop

Ensure that the vhost-user slave knows when the vrings are valid and
when they are invalid, for example during a guest reboot.

The vhost-user protocol says this of VHOST_RESET_OWNER:

      Issued when a new connection is about to be closed. The Master
      will no longer own this connection (and will usually close it).

Send this message to tell the vhost-user slave that the vhost session
has ended and that session state (e.g. vrings) is no longer valid.

Signed-off-by: Luke Gorrie <luke@snabb.co>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>