Fix hardcoded Xen kernel path

We already have a ./configure-time variable for this, but it seems to
be actually unused.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
(cherry picked from commit 3c4afa2e)

Signed-off-by: Iustin Pop <iustin@google.com>
(trivial patch, let's cherry-pick it)
Reviewed-by: Michael Hanselmann <hansmi@google.com>