configure: Fix xen probe with Xen 4.2 and later

The xs.h header is now deprecated and produces a warning. This prevents the
configure script from enabling Xen with xen-unstable whom will become 4.2. As
this header is not anymore common to every version of Xen, we just remove it
from the early probe for Xen.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
