sockets: avoid formatting buffer that may not be NUL terminated

The 'sun_path' field in the sockaddr_un struct is not required
to be NUL termianted, so when reporting an error, we must use
the separate 'path' variable which is guaranteed terminated.

Fixes a bug spotted by coverity that was introduced in

  commit ad9579aa
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Thu May 25 16:53:00 2017 +0100

    sockets: improve error reporting if UNIX socket path is too long
Reviewed-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Message-Id: <20170626103756.22974-1-berrange@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

util/qemu-sockets.c

@@ -897,7 +897,7 @@ static int unix_listen_saddr(UnixSocketAddress *saddr,
     strncpy(un.sun_path, path, sizeof(un.sun_path));
 
     if (bind(sock, (struct sockaddr*) &un, sizeof(un)) < 0) {
-        error_setg_errno(errp, errno, "Failed to bind socket to %s", un.sun_path);
+        error_setg_errno(errp, errno, "Failed to bind socket to %s", path);
         goto err;
     }
     if (listen(sock, 1) < 0) {