vchan-socket-proxy: Move perror() into listen_socket

vchan-socket-proxy: Move perror() into listen_socket

The use of perror on the return from listen_socket can produce
misleading results like:
UNIX socket path "/tmp/aa....aa" too long (156 >= 108)
listen socket: Success

errno is reset by subsequent system & library calls, so it may be
inaccurate by the time listen_socket returns.  Call perror immediately
after failing system calls to print the proper message.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Wei Liu <wl@xen.org>
Reviewed-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Release-acked-by: Paul Durrant <paul@xen.org>