virtfs-proxy: Fix possible overflow

It's detected by coverity. The socket name specified
should fit in the sockadd_un.sun_path. If not abort.

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>