vhost-user: fix shared object return values

vhost-user: fix shared object return values

VHOST_USER_BACKEND_SHARED_OBJECT_ADD and
VHOST_USER_BACKEND_SHARED_OBJECT_REMOVE state
in the spec that they return 0 for successful
operations, non-zero otherwise. However,
implementation relies on the return types
of the virtio-dmabuf library, with opposite
semantics (true if everything is correct,
false otherwise). Therefore, current
implementation violates the specification.

Revert the logic so that the implementation
of the vhost-user handling methods matches
the specification.

Fixes: 043e127a126bb3ceb5fc753deee27d261fd0c5ce
Fixes: 160947666276c5b7f6bca4d746bcac2966635d79
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
Message-Id: <20241022124615.585596-1-aesteve@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>