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: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20170626103756.22974-1-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>