fsdev/virtfs-proxy-helper: Fix improper use of negative value

It's detected by coverity. Check the return value of proxy_marshal.

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
