hw/virtio/vhost-user: avoid using unitialized errp

During protocol negotiation, when we the QEMU
stub does not support a backend with F_CONFIG,
it throws a warning and supresses the
VHOST_USER_PROTOCOL_F_CONFIG bit.

However, the warning uses warn_reportf_err macro
and passes an unitialized errp pointer. However,
the macro tries to edit the 'msg' member of the
unitialized Error and segfaults.

Instead, just use warn_report, which prints a
warning message directly to the output.

Fixes: 5653493 ("hw/virtio/vhost-user: don't suppress F_CONFIG when supported")
Signed-off-by: Albert Esteve <aesteve@redhat.com>
Message-Id: <20230302121719.9390-1-aesteve@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 90e3123)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/virtio/vhost-user.c

@@ -2108,8 +2108,8 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
         } else {
             if (virtio_has_feature(protocol_features,
                                    VHOST_USER_PROTOCOL_F_CONFIG)) {
-                warn_reportf_err(*errp, "vhost-user backend supports "
-                                 "VHOST_USER_PROTOCOL_F_CONFIG but QEMU does not.");
+                warn_report("vhost-user backend supports "
+                            "VHOST_USER_PROTOCOL_F_CONFIG but QEMU does not.");
                 protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
             }
         }