virtfs-proxy-helper: fail gracefully if socket path is too long

Replace the assertion check with graceful failure when the socket path
is too long.  Programs should not crash on invalid input.  Print an
error message and exit properly.

Cc: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
