x64/xen: Fix assignment to xen_features.

The code wouldn't work if XENFEAT_NR_SUBMAPS > 1. It's currently 1.
The assignment to xen_features must take i as well as j into consideration.

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