xen: remove designated initializer in C++ code

While (unfortunately) C++ doesn't support designated initializers, and the
compiler rejects them, one instance has survived in xenbus.  Strangely,
gcc 4.8.2 generates correct code, while gcc 4.8.0 fails with an internal
compiler error, instead of both of them rejecting the code.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>