xen: Fix xen_features initialization

Ironically, my commit 0c77d3c broke xen_features initialization.
This patch fixes it.

Reported-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphael.scarv@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>