vhost-backend: stick to -errno error return convention

vhost-backend: stick to -errno error return convention

Almost all VhostOps methods in kernel_ops follow the convention of
returning negated errno on error.

Adjust the only one that doesn't.

Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
Message-Id: <20211111153354.18807-7-rvkagan@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>