vchan-socket-proxy: Ensure UNIX path NUL terminated

vchan-socket-proxy: Ensure UNIX path NUL terminated

Check the socket path length to ensure sun_path is NUL terminated.

This was spotted by Citrix's Coverity.

Also use strcpy to avoid a warning "'__builtin_strncpy' specified bound
108 equals destination size [-Werror=stringop-truncation]" flagged by
gcc 10.

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>