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

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>